Hmm, AT is already full of lots of fantastic creatures, converting them ALL to ponies would be both futile and silly. The humanoids like Princess Bubblegum, Marceline, or the Ice King would make sense, but characters like Peppermint Butler, not so much.
Anyway, nice drawing, though I'm curious why you didn't give him a blue coat and axe the clothing (keeping the hat, though). Or, for that matter, swap the backpack for saddlebags. It does look good the way it is, though.