When Siph tags along with me to Equestria, he has to don a disguised form, too, because a Yoshi in Equestria would stand out just as much as a human. And I prefer not to draw unnecessary attention to myself when travelling dimensions just because of what I am instead of what I do.
At first I suggested he be a little dragon, because Yoshis are described as dragons sometimes, but Siph told me in no uncertain terms that he wanted to be an Earth pony colt. So I relented and let my little Yoshi be a pony. It had the unexpected benefit of giving him a ears for me to grab when he acts up. Yoshis don't have those! He also still has his watch too. We can't transform or keep our forms without our watches!

He's still the most adorable thing out there.