The character Poochy did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Poochy is Yoshi's pet dog who follows Yoshi throughout the Yoshi's Island series. You can leap on his back to scale spikes, or guide him to hard to reach places to get daisies or yarn.
Poochy is a medium-sized brown and white dog with an abnormally large mouth, with a large tongue. He also has small, black eyes, and a tail with a circle that sticks out on the end. He wears a black collar with gold buttons on it.
In Yoshi's Island U, Poochy does not appear until act 1 of the plot is finished. Once it is, Poochy gets his own levels, with no Yoshi. Then, Poochy is transported twenty-five years in the future, along with all of Yosher's Isle and surrounding area.