The Sixth Dino
My first exposure to Doctor Who, like most Americans of my age range, was the Fourth Doctor (Tom Baker), since he had the longest run of any actor on-air and was shown most often on PBS here in the US. But I’d say the Sixth and Seven incarnations probably made more of a lasting impression on me when I watched the show as a kid.
I don’t remember much of specific adventures of the Sixth Doctor, but I do recall him seeming to have a manic quality to him, especially soon after he regenerated. And it was from this impression of him that I thought a Velociraptor would fit him best.