They are legal, as long as the people you play with don't mind. This is, first and foremost, a game. If everyone is having fun and enjoying themselves, you can play any card you want. If you can figure out how to make a Pikachu or a Red Eyes
Black Dragon work, and the people you play with don't mind, by all means play them. I'd be down for the other thumb and the D&D Sword at my table, and so would most people I've played against.