A Hacker Yoshielectron on youtube or James S. has found the yoshi egg texture buried in wet dry worlds memory. Although he's not the first one to do this. He gave it the animation of a coin, but it normally is just a texture with no animation. Heres is his video:
In this games sequal, Super Mario Sunshine, you could hatch an egg that looks exactly like the hidden yoshi egg texture buried in the game's memory. In sunshine you could ride yoshi after hatching the egg, but yoshi would die/disappear if you went into the water. Since wet dry world is a water level, the same mechanics could be used if yoshi was rideable. This leads us to think that yoshi was planned to be rideable in SM64. This is another huge SM64 mystery. What does the yoshi egg texture buried in wet dry world's memory mean? What does the star statue in the courtyard mean? How many licks does it take to reach the center of a tootsie pop? The world may never know.