There are several Welsh proverbs which relate to the Gelert story.
Yr wy’n edivaru cymmaint a’r Gwr a laddodd ei Vilgi.
(I repent as much as the man who slew his greyhound.)
Jones, E., Musical and Poetical Relicks of the Welsh Bards, (second edition, doubly augmented and improved, 1794), p. 75 [and in 3rd edition, (1808), vol. 1, p. 75; 4th edition 1825]
Repeated in Jacob, Joseph, Celtic Folk and Fairy Tales
Happy is the man who never killed his greyhound.
Sotheby, William?, A Journal of a tour through parts of Monmouthshire and Glamorgan, [1805?], NLW 6497C, pp. 48b-49 when at Abergavenny
Mor ffôl â’r gŵr a laddes ei filgi.
As foolish as the man who slew his greyhound.
Crawford, Mrs, Hugh Lloyd, The Warrior and Bard of Cambria, A Welsh Tale, The Metropolitan Magazine, Volume 29, (1840), p. 302
Cyn dial gwybydd yr achos. Ystyr ddwywaith cyn taraw unwaith
(Before Revenge, first know the cause. Reflect twice before striking once.)
Da yw ffrwyno llid rhag dialu yn angyfiawn, fal y gwnaeth y gwr a laddwys ei Filgi
(It is well for a man to bridle his rage, lest he should avenge himself unjustly, like the man who killed his Greyhound.)
From Iolo Morganwg’s version of the Gelert story. 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. 155 in Welsh; p. 561 in English
Dorson, Richard Mercer, History of British Folklore, Volume 1, (London : Routledge, 1968, reprint 1999), pp. 429
The following come from a long list of proverbs:
Bu mor edivar ganddo a’r gwr à laddoedd ei vilgi.
Mor edivar gantho a’r gwr à laddoedd ei vilgi.
The Myvyrian archaiology of Wales, vol. 3 (1807), pp. 150, 165
The Myvyrian archaiology of Wales collected out of ancient manuscripts by Owen Jones (Myvyr), Edward Williams (Iolo Morganwg), William Owen Pughe (Idrison); … vol. 3, (2nd ed., Denbigh : Thomas Gee, 1870), pp. 841, 851,