I don’t usually get too attached to Animals or Pets in Anime, but I recently watched Servamp, so I’d say Lawless of Greed. I think this counts anyway, he does has an Animal Form.
I also like Kuro (Sleepy Ash of Sloth). I love that they are based on the Seven Deadly Sins, It’s really interesting, and they have personalities to match.
Do you have a Favourite Animal Sidekick, Pet, or Summoning from an Anime? Have you watched Servamp?