That he is not in Smash Bros. (yet) but he is in this Indie game instead.
Megaman was in street fighter x Tekken and also was in smash bros... And megaman!, no reason Rayman couldn't be in more than one game.
Also smash isn't the only game with guest fighters, soul caliber has the Witcher, Tekken had gon, street fighter has guy/Cody. Etc.
Fancy hearing me on an amateur podcast with friends gushing over one of my favourite games? https://youtu.be/1I7JfMMxhf8