Perhaps it depends if you have dogs or not. I have three dogs and they are definitely WHO. But I agree with SuKi (she's usually right so I have to agree!) that Which dog is Coco? would be the more usual way of putting it (if there is a group). In general, Who is Coco? is fine to use as a question, especially if you just hear the name 'Coco. "Who is Coco?" "Coco's a dog." "Oh, Coco's a dog."

It would be strange to use 'who' for an animal. You might say 'Whose dog is Coco?' ('He's my dog') Or if you're looking at a group of dogs and want to identify Coco, you'd say 'Which one is Coco?'

yes we cant use "who" for an animal but i have a doubt if i can use "which"?