I'm stumped. It's certainly not a peacock, but it doesn't look like a calf, either, and the legs are too short for a baby giraffe. If it's a hare, then the nose is unusually long.

I'm almost certain it is a peacock, though it took me a long time to understand. It's critical that this was made in the 5th century, when artisans weren't necessarily understanding the pagan scenes they were supposed to depict. I think the "proof" lies in numismatics. One of Juno's standard poses is her standing with a stick in one hand, pouring from a plate in the other - with a peacock tilting its head up in order to drink the water.



The funny thing is that this artisan did not understand he was copying a peacock. He gets the beak and feathery body right, but he turns the feathers that hang down from the bird's head into big ears.



This is where I'm getting a bit speculative, but it looks to me like the final product is influenced by art the Egyptians saw around them. Like this Set animal: