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!