I was just 100% speedrunning Super Mario World on the GBA for practice, I was in the Forest of Illusion secret area, where I hit the ? block and out came a pale looking yellow Yoshi, with bright-brown shoes, and a green saddle. At first, I didn't think much of it. But later, I came to realize that no matter I shell I had in Yoshi's mouth, I had all the additional effects (I.E. fire balls, stomp, and flight). Sadly, my game crashed after attempting to save. So, I implore you to try to find out what this is and how to trigger it, if possible. I can't reward you much, but I mean the adventure must be fun! I myself have been digging relentlessly, but no dice... I figured it could have something to do with power implementation, as the power up effect is not normally seen normally in both scenarios. That may be a lead, I don't know.

This sounds like a job that is much more in your own wheelhouse of expertise. Next time be sure to record what you find. :)