I vaguely knew of the game but not the character. Do you know who this guy is?
It's the Prince of the Sablé Kingdom, of course! (Who?) He's from Intelligent Systems' Japan-only Kaeru no Tame ni Kane wa Naru, which released for Game Boy in 1992.
You might just know it as The Frog For Whom the Bell Tolls or For the Frog the Bell Tolls -- we've covered a fan-translation effort before. This top-down action/adventure game is reminiscent of Zelda and, yeah, the rest of the world missed out by not getting it.
In his announcement post, director Masahiro Sakurai says that for the Prince's appearance in Super Smash Bros., "He transforms into a snake or a frog, and unloads multiple attacks on any opponent he catches. Keep attacking your foes when they're trapped in his dust cloud."
The model for the frog prince is beyond adorable. Those little boots!