Squirtle is Fushigisou, Ivysaur is Venigame, and Charizard is Lizardon. A quick google search or trip to Serebii could have revealed the same.
You also forgot Purin for Jigglypuff
I meant why are they called that in the file names.
. . .
Those are the Japanese names of the Pokemon. .-. Pokemon is a creation of Japanese developers. Smash Bros. is a creation of Japanese developers. Put two and two together, and it makes sense for the file names to be Japanese.