The Abergarwan story

This comes from a manuscript which the great collector (and forger) of manuscripts, Edward Williams, better known as Iolo Morganwg, (1747-1826) claimed to have found amongst the ‘Fables of Cattwg the Wise’, otherwise known as St Cadog. One of the thirteen fables (12 in the hand of Iolo, one in the hand of his son Taliesin) includes a version of ‘The Man who killed his Greyhound’. We do not know whether Iolo concocted these stories, adapted them from stories he knew of from other sources, or copied them from earlier documents which no longer survive. He knew of the Panchatantra version of the story as shown in a letter he wrote in 1800:
In the Heetopadees the fable of the ‘Bramin and the jihneumon, is precisely the same as that of Y Gwr a lladod ei filgi [The man who killed his greyhound].
Letter from Iolo Morganwg to Owen Jones (Owen Myfyr), 17.6.1800. Jenkins, Geraint H.  et al, The Correspondence of Iolo Morganwg, vol. 2, 1797-1809,  (University of Wales Press, 2007), pp. 288-289

The story is set in a place called Abergarwan. Its location is unknown, but it is supposed to be in south Wales (possibly near where Iolo lived). There ia a place called Abergarfan in the Corris valley

Welsh version
‘Dammeg y Gwr a laddwyn ei filgi’

English version
Collection of ‘Fables of Cattwg the Wise, (son of Gwynlliw … ) no. 2 (of 12) ‘The Man who killed his Greyhound’.
“There lived formerly at Abergarwan, a man and wife, who had a son, and he was their only child, an infant in his cradle. One day, when his wife was gone to attend her devotions, the man heard the cry of hounds on his land, in full chase after a stag. ‘I will go to meet them,’ said he, ‘that I may, as lord of the land, get the share due to me of the stag.’ And away he went, leaving his child in the cradle, and near the cradle lay his Greyhound. Whilst the man was absent in the field, a Wolf entered the house, and would have killed and devoured the child ; but the Greyhound fought hard with the Wolf; and after a long and bloody struggle, and many wounds and bruises, he at last succeeded in killing him. It so happened that during the struggle the cradle was by some means or other overturned, and it lay on the ground with its face downwards. When the man returned to the house, the Greyhound, covered with blood, got up to welcome his master, and showed symptoms of joy at his return by shaking his head and wagging his tail. But the man, when he discovered blood on the Greyhound and a pool of blood on the floor, thought that the Greyhound had killed his only child; and so, in a fit of rage and distraction, he thrust the Greyhound through with his sword, and killed him. But when he went to the cradle, and had turned it up, and found his child alive and unhurt, and saw the Wolf lying dead by the side of the cradle, and that the Greyhound had been mangled and torn by the teeth of the Wolf, he became almost frantic with grief. Hence arose the proverbs: ‘ Before revenge, first know the cause,’ and ‘ Reflect twice before striking once.’ This circumstance gave rise to the following expressions : ‘As sorry as the man who killed his Greyhound’ ; ‘A hasty act is not a prudent act, but like the man who killed his Greyhound.'”

Sources:

Damegion Cattwg Doeth, [c. 1800-1825].
A notebook containing ‘Dammegion Cattwg Ddoeth ap Gwynlliw ap Glywis ap Tegid ap Cadell Deyrnllwg’ [See Iolo Manuscripts (Liverpool, 1888), pp.154-65]. The first twelve are in the autograph of Iolo Morganwg, while the thirteenth, ‘Dammeg Arthur a’r Hanner Dyn (Gan Taliesin)’, has been copied by Taliesin Williams from a loose sheet in Iolo’s handwriting which has been inserted immediately after the copy referred to.
NLW MSS 21363-4B.
1842
Miscellaneous ancient Welsh manuscripts : with translations and notes, by Taliesin Williams, (Ab Iolo) ; published for the Welsh MSS. Society. (Llandovery : William Rees, 1842), pp. 154-155
1848
Anon [Williams and Williams] (Editors), Iolo manuscripts : a selection of ancient Welsh manuscripts, in prose and verse, from the collection made by the late Edward Williams, Iolo Morganwg [1747-1826], for the purpose of forming a continuation of the Myfyrian archaiology; and subsequently proposed as materials for a new history of Wales with English translations and notes, by his son, the late Taliesin Williams, (Ab Iolo) … Re-edited and published for the Welsh mss. society. (Llandovery, 1848), p. 154 in Welsh; p. 561 in English
1861
A Welsh language version was published by William Jones in his prize winning essay on the Parish of Beddgelert, in Y Brython, vol. 4, (1861), p. 129, and reproduced in Plwyf Beddgelert (1862), p. 13,
1888
Iolo manuscripts : with English translations and notes, by his son, the late Taliesin Williams, (Ab Iolo), (Liverpool : I. Foulkes, 1888), pp. 154-155; 561
1899
Jenkins, D.E., Bedd Gelert: Facts, Fairies and Folklore, (1899), pp. 57-58

Notes on folklore in the collection of Edwin Sidney Hartland, (1848-1927), sometime President of the Folk-lore Society includes on first page a transcription from the Fables of Cattwg the wise … no 2, The Man who killed his greyhound from Iolo’s published version.
NLW 6817E. (HARTLAND 27)

The following manuscripts, from Iolo Morganwg’s collection, contain items relating to Cattwg Ddoeth (Cattwg the Wise), but do not appear to include the story of ‘The Man who killed his Greyhound’.

NLW MS 13118B, pp. 207-390 (previously paginated 1-184) contain a collection of maxims, proverbs, triads, sayings, etc., attributed to Cattwg Ddoeth and described on a ‘title-page’ to the section ( p. 199) as ‘Llyma Ddoethineb Cattwg Ddoeth o Lancarvan’ and in a concluding note (p. 390) as ‘Llyfr y cyntaf y Gwyddfardd Cyfarwydd’. In a note on the aforementioned ‘title-page’ (p. 199) Edward Williams claims to have transcribed this collection in 1799 from a manuscript in the possession of Siams Thomas of Maerdy Newydd, co. Glamorgan. Preceding and following the actual text of the collection are transcripts of a prefatory letter dated 1685 (pp. 201-06) and of the concluding note already referred to (p. 390) both of which are attributed to the Glamorgan scribe and copyist Thomas ab Iefan of Tre Bryn as compiler of the manuscript from which Edward Williams was allegedly copying (see TLLM, t. 172; IM, tt. 291-4).

NLW ms 13093E
p. 13 copies of ten tales or fables in Welsh, nine bearing the titles ‘ Dammeg y Dial’, ‘Dameg y Ceiliog Rhedyn a’r Moryn’, ‘Dammeg y Dylluan, y golomen, a’r ystlym’, ‘Dameg y geifr, y Defaid, a’r bleiddiaid’, ‘Dameg y march gwyllt’, ‘Dammeg yr Eos a’r hebog’, ‘Dammeg Cenfigen yn Llosgi ei hun’, ‘Dammeg y Gwr a’r [Ebol]’, and ‘Dammeg Meredydd ap Rhosser o Lanbedr a’r Fro am gastell Tre Warin’, and the tenth telling the story of Tanwyn, the son of Trahaearn, the bard (for the Welsh text of nine of these see Iolo Manuscripts . . ., pp. 167-84, and for English translations ibid., pp. 577-96)

NLW ms 13118B BARDDONIAETH; DOETHINEB CATWG DDOETH. [all in Welsh]
Pp. 207-390 (previously paginated 1-184) contain a collection of maxims, proverbs, triads, sayings, etc., attributed to Cattwg Ddoeth and described on a ‘title-page’ to the section (p. 199) as ‘Llyma Ddoethineb Cattwg Ddoeth o Lancarvan’ and in a concluding note (p. 390) as ‘Llyfr y cyntaf y Gwyddfardd Cyfarwydd’. In a note on the aforementioned ‘title-page’ (p. 199) Edward Williams claims to have transcribed this collection in 1799 from a manuscript in the possession of Siams Thomas of Maerdy Newydd, co. Glamorgan. Preceding and following the actual text of the collection are transcripts of a prefatory letter dated 1685 (pp. 201-06) and of the concluding note already referred to (p. 390) both of which are attributed to the Glamorgan scribe and copyist Thomas ab Iefan of Tre Bryn as compiler of the manuscript from which Edward Williams was allegedly copying (see Traddodiad Llenyddol Morgannwg (Caerdydd, 1948), t. 172; Iolo Morganwg. Y Gyfrol Gyntaf (Caerdydd, 1956), tt. 291-4). Pp. 391-477 contain a transcript of a collection, in alphabetical order, of over three thousand Welsh proverbs attributed to Cattwg Ddoeth (‘Llyma Ddiarhebion Cattwg Ddoeth . . . sef yw hwnn Ail Lyfr y Gwyddfardd Cyvarwydd’). This collection, according to the aforementioned note at the end of the preceding section (p. 390) attributed to Thomas ab Iefan, had been compiled by the said Thomas from various sources and formed a continuation of the previous section. Edward Williams’s claim with regard to the Siams Thomas volume is probably intended to apply to the contents of pp. 391-477 as well. The contents of pp. 199-390 have been published in The Myvyrian Archaiology of Wales . . ., vol. III (London, 1807), pp. 1-99. For the proposed publication of the contents of pp. 391 477, also in The Myvyrian Archaiology, see the footnote to IM., tt. 291-2.

NLW ms 13157A
In the hand of Edward Williams (‘Iolo Morganwg’), a prefatory note to an intended collection of proverbs, moral aphorisms, etc., allegedly composed or compiled by Cattwg Ddoeth.
p. 73-74
‘Preface to Cattwg Ddoeth. A few short pieces in this collection are in some mss. attributed to Taliesin and it is not only possible but also probable that they are Taliesin’s without any impeachment of the authenticity of the work as attributed to Cattwg who should be considered as the collector or compiler rather than the author or writer, not but that it is highly probable that he is also the author of many of the proverbs and moral aphorisms. …
on p. 32 he wrote:
Cattwg the Bard of the primitive school, simple verse, subjects morality Religion etc. all his verse moral.

1906

THE MAN WHO KILLED HIS GREYHOUND (A WELSH STORY). There lived formerly at Abergarwan a man and wife who had a son, and he was their only child, an infant in his cradle. One day when his wife was gone to attend her devotions, the man heard the cry of hounds on his land, in full chase after a stag. “I will go and meet them.” said he “that I may as lord of the land get the share due to me of the stag.” And away he went leaving his child in the cradle and near the cradle lay his greyhound. Whilst the man was absent in the field, a wolf entered the house and would have killed and devoured the child: but the greyhound fought hard with the wolf and after a Iong and bloody struggle, and many wounds and bruises, he at last succeeded in killing him. It so happened that during the struggle the cradle was by some means or other overturned, and it lay on the ground with its face downwards. When the man returned to the house, the greyhound, covered with blood, got up to welcome his master and showed symptoms of joy at his return, by shaking his head and wagging his tail. But the man when he discovered blood on the greyhound and a pool of blood upon the floor thought that the greyhound had killed his only child, and so, in a fit of rage and distraction, he thrust the greyhound through with his sword, and killed him. But when he went to the cradle, and had turned it up and found his child alive and unhurt, and saw the wolf lying dead by the side of the cradle, and that the greyhound had been mangled and torn by the teeth of the wolf, he became almost frantic with grief. Hence arose the proverb. “Before revenge, first know the cause” and “Reflect twice before striking once.” This circumstance gave rise to the following expressions. “As sorry as the man who killed his greyhound.” ‘”A hasty act is not a prudent act: but like the man who killed his greyhound.” The man who suffers his passion to get the better of his prudence will commit an act which he will never be able to undo and as long as he lives, it will cause him painful sorrow. It is well for a man to bridle his rage. lest he should avenge himself unjustly, like the man who killed his greyhound. The above is a translation of a very old Welsh fable. It would add to our knowledge to know whether the Abergarwan of the fable is the same as the present Beddgelert of North Wales. It may be that the poem entitled Gelert’s grave was composed after reading the Welsh fable or hearing it repeated. If so an old tradition is destroyed. C. School, Alltwalis. R. E. WILLIAMS.
The Carmarthen Journal and South Wales Weekly Advertiser, 24th August 1906

1955 onwards
The Abergarwan story was the basis of one of the most popular versions of the story, as told by Gwyn Jones. There is no mention of Gelert or Beddgelert in this version.
[Summary] The story of an unnamed lord of Abergarwan, who had an only infant son. While his wife was at her devotions, the lord heard the hunting horn and saw a spent stag being chased by hounds and huntsmen, and joined them, indicating to the hound that he should stay to guard the baby. A wolf walked in, wishing to devour the child. The wolf and hound fought, upsetting the cradle and splashing the blankets with blood, but the child did not wake. The wolf was killed by the hound, who greeted the lord on his return, but when the lord saw the blood and upset cradle, he killed the hound with his sword. As the hound died, the baby began to cry and the lord found him unharmed in his silk sheets, and saw the dead wolf and realised what had happened. Grief pieced his heart, but nothing would restore his hound to life. ‘He told a bard to make a story of his haste and folly and he had the dog buried in a high place like a hero. The grave is long lost but the story remains with a proverb which grew out of it ‘As sorry as the man who killed his greyhound.
Jones, Gwyn, Welsh legends and folk-tales retold by Gwyn Jones; ([London] : Oxford University Press, 1955) (and further editions in 1957, 1979, 2001)
Jones, Gwyn, ‘The man who killed his greyhound’, Tales from Wales, no. VIII, v., pp. 235-237, (Puffin, 1979)