If Buddha is an incarnation then how do we understand his previous incarnations as animals mentioned in the Jataka tales?

From: rishit

Hare krishna prabhuji.Please accept my humble obeisances. my question is that :-  we know that Buddha was an incarnation of Lord  Vishnu. Yet from the Jataka tales we come to know that Buddha reveals he came in his earlier lives as a deer , a thief, a dog and many more.Is the information authentic. Again if this were true then in what categories do these avataras as a deer, theif or dog . Are they mentioned in shastras.