To be fair, I hardly had to do anything... It was an existing character, a background pony used for crowd scenes that the fandom adopted and named "Doctor Whooves" for his supposed resemblance to a certain Time Lord character.

When they actually featured him and various other background characters that the fandom had named and adopted in the 100th episode of the series, they threw in plenty of Doctor Who references, but mixed in a good deal of Doc Brown from Back to the Future as well. As a result, he's something of a super-scientist:



I really only needed to add some blonde highlights, a goatee and the trademark bubble pipe.