Yoshi was originally meant to be a Koopa

[Coolblue]

(Reposting this because I have the proof of the booklet)

In the Super Mario History booklet that came with the Super Mario 25th Anniversary Collection, Takazi Tezuka, who was involved with the creation of the Mario and Zelda series, stated that Yoshi was originally meant to be a type of Koopa in Super Mario World. He also states that the saddle on Yoshi's back is actually his shell.

Quote: Tezuka: Yoshi was originally supposed to be a type of Koopa. That saddle you see on his back was actually his shell.