The taming and yolking of stags is a common theme in Welsh saints lives. The "Celtic Saints", by and large, have a good relationship with wild animals, taking their cue more from Thecla of Iconium than Ignatius of Antioch.
Many of the saints are credited with taming wild animals. The best animal tamer is St Kieran (English Translation), who befriended a boar, badger (or beaver?), wolf and doe, along with a brogue-thieving fox. In most cases these are otherwise dangerous creatures: in the Life of St Tatheus (English Translation), a swineherd (of the saint?) complains that a wolf has eaten his piglets. After Tatheus' prayers the she-wolf leaves one of her cubs with the swineherd, which grows into a guard dog, thereby repaying the debt. Equally, St Patrick turned a vicious dog, set upon him by stone-worshiping Pagans, as still as stone (English Translation).
Stags, also not to be messed with, feature most heavily. St Oudoceus instantly tames a stag which is being hunted by King Einion of Glewyssig, who then gives Oudoceus all the land traversed by the stag that day (English Translation). This matches the popular medieval tale of St Giles, the 7th C. saint of cripples who was wounded while protecting a deer from a king's hunters (St Giles is usually portrayed with the deer) (Info). In the Life of St Tatheus (English Translation), the saint sails to Gwent, and on arrival is invited to bathe. While doing this, a servant is sent to secure his boat, but finds its rope held hast by the feet of a stag. On one occasion St Patrick was so piteous of a fawn that he saved both it and its mother, who followed him tamely, placing the fawn in a park in Ardmachia (English Translation). However, not all animals were so fortunate, and many are called upon to make the ultimate sacrifice in the Lives. Patrick once commanded two stags and two boars from a wood, which he then had killed for food (English Translation). On a milder level of consumption, at the church of Ath-da-laarg, county Meath, St Patrick founded a church and left in charge of it three brothers, plus a sister who milks a deer (English Translation). Equally, another doe is tame enough to milk at Telac-na-licce - the deer also marking out the location of the church there (English Translation).
Given that many of the Lives were written in the 12th C. it seems unlikely such familiarity is a late addition under the influence of, for example, St Francis (1182-1226; Info). It is more likely that the hagiography of Francis is simply an example of how wide-spread these themes were. That there is somewhat more going on here is indicated by the strong relationship between guiding animals and the saints in the "Celtic" Lives (Note) and the tale from one version of St Patrick's life (English Translation) in which Patrick uses a cloak of invisibility to disguise himself and his fellows as a group of deer.
With regards the yoking of wild animals, examples include St Tydecho, who, after King Maelgwn steals his oxen, ploughs with stags and then a grey wolf* and St Illtud (See Life). In the 12th C. Life of St. Cadog (English Translation), we have the story of how two of his students, incorrectly accused of laziness, order two wild stags to the yoke to aid them (hence the name of the place Nancarbania, from Nant Carwan "Valley of the Stag" [Carw]). This is mirrored in the Life of Teilo (English Translation), in which St Teilo and Maidoc gain two yoked stags to help them. These animals then attract more people to their church through their miraculous taming.
In both the life of St Deiniol (English Translation)
and the Life of Kentigern
two stags come from the local wood to be yoked when the saint can't find animals to plough with. In Kentigern's case
a wolf eats one of the stags, and Kentigern demands it to the yoke to finish the job along with the remaining stag!
This may be the broader origin of the St Tydecho nugget, above.
Jocelyn, author of the Kentigern Life, relates this to Isaiah 11:6:
The wolf will live with the lamb, the leopard will lie down with the goat,
the calf and the lion and the yearling together; and a little child will lead them [NIV Version]. Green* notes that the
Aberdeen Breviary includes the story of St Fillan, who "after he had poured out his prayer to God, the same wolf coming back as a tame creature, submitted itself to the yoke of the team along with the remaining oxen".
In a number of cases, the yoked animals draw the body of the saint to its resting place, having mysteriously appeared. As well as the example of St Tewdrick (See The Returning Plank), in the Life of St Cellach (English Translation), his body is refused burial in several locations, until two deer approach with a cart carrying a bier. This they lay on the ground for the body, then carry the body on it to Eskers' church, where the bells pealed on their own. The deers hung round after the burial, ploughing church ground and licking the saint's tomb. In one version of St Patrick's life (English Translation) his body is also carried away to Armagh by two stags on a cart after humans argue over who is to get it. However, the animals are as often bulls: in another version of Patrick's Life it is two tame oxen, who are allowed to wander with the body to Dun-da-leth-glas (English Translation), while in yet another the two oxen stop at Dunum, while a phantom cart and oxen entice the people of Ardmachia, who are desperate for the body, to the river Caucune, where it disappears (English Translation). In the Life of Kentigern (English Translation) the saint pulls the body of Fregus in a cart which he yokes to two untamed bulls. This has been regarded by some as a metaphor or pointed reference to Christianity bringing the gods of the Pagans under their control*.
Finally, in opposition to the above examples, it should be noted that tradition associated with St David suggests he was both vegetarian, and insisted his monks pull their own ploughs. Quite right too...