After the development of Super Mario Bros. finished, which is when the development of Super Mario Bros. 3 started, Miyamoto said he wanted Mario to ride a horse. He had drawn a picture of Mario on a horse, and hung it on a wall near where he used to sit. Takashi Tezuka would look at that and think, "I think he wants Mario to ride something." The concept art sat there for 5 years (from the beginning of Super Mario Bros. 3's development to the beginning of Super Mario World's development.) Due to technical limitations of the Famicom, the idea was shelved until Super Mario World started development. The “horse” aspect impacted how Yoshi was first drawn by Shigefumi Hino (by being a large creature) during the development of Super Mario World.
On July 24, 2020, a vast amount of source code from Nintendo's early consoles surfaced as well as prototypes and assets from games such as Yoshi's Island, Super Mario Kart, The Legend of Zelda: A Link to the Past, and more.
The leak included a portion of Super Mario World's source files with an absurd amount of early assets. While some had previously been found in the SNES Test Program and SNES Burn-In Test Cart or had been glimpsed in grainy magazine screenshots, many had been completely unknown and show how significantly the game changed over its development cycle.
Among these files were possibly the earliest versions of Yoshi, taking on a very lanky and primitive appearance. These files are dated 12 July 1990 10:44. Within these files additional graphics for Mario riding this version of Yoshi can be found, along with graphics of Yoshi spitting fire.