Speaking of dogs:
In the 13th century in France, a dog named Guinefort was venerated as a saint. Locals would travel to his grave, where they claimed miracles would occur, especially over infants.
The legend went as follows:
A certain knight left his infant son in the care of his greyhound Guinefort while he went hunting. When he returned, he found the child’s cot overturned, the dog and room smeared with blood, and the child nowhere to be found. Enraged that his previously loyal hound had eaten his only child, the knight drew his sword and slew Guinefort on the spot. Immediately thereafter, the knight’s wife turned the cot back over, only to find the child laughing and unharmed, and the bloody corpse of a viper lying next to him.
The knight repented of having killed such a loyal friend and lowered the dog’s body into a well, which he filled in with rocks and converted into a shrine. When the people heard the story, they called the dog a martyr and venerated him as a saint who performed healing miracles, particularly among young children.
(This tale is very similar to the story of the Welsh hound Gelert, except with a wolf instead of a snake, and the added detail that the dog’s owner—Llewellyn the Great—was forever haunted by the dying yelp of the dog and never smiled again.)
Guinefort was never recognized as an official saint, and the church tried to suppress his cult. In fact, the inquisitor Stephen of Bourbon was SO MAD at the idea that someone would say a dog could be a saint, he made it clear that the cult of Guinefort was IN FACT a secret devil-worshiping operation and all babies that were allegedly healed by a dog’s ghost were actually eaten by Satan. He had the shrine destroyed and the body and relics of Guinefort burned for heresy.
To reiterate: under church law, it is impossible for a dog to be a saint; one can, however, be a heretic.
Despite the efforts of killjoys like Stephen of Bourbon, the cult of Guinefort lasted for centuries, well into the 1930s.
As far as I know, there have been no other dog saints. Except for Saint Christopher, of course, who we all know was half dog.