Yoshi was originally meant to be a Koopa

[Coolblue]

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 Super Mario 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.

Edit: Pic of the book with the quote from Mr. Tezuka.

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