I believe it's because he's a non-pony creature.

The exact difference between a non-pony creature and a minor character isn't set in stone though, so some moderators may put such characters here, others may put them in Minor Characters.

They are kinda hard to get, considering that all we have are slightly distorted and completely sepia'd memories of Granny. Although, I am fairly sure I have seen at least one timberwolf vector around here somewhere...

Do you know where otters appear in the show? Maybe it would be possible to request it.