Only one, not counting things like Gyarados. A Caterpie in the HGSS Bug-catching contest. I just happened to randomly run into it. Needless to say, I immediately captured it and didn't give a crap about losing the contest :p
I've gotten random Pokerus at least twice.
Hidden details about the Wii U Yoshi game I noticed: http://www.gamefaqs.com/boards/631516-wii-u/65259642