Dig-Dug and Pooka were based on how they look in the official art and select screen. Fygar was based on a mix of the official art, but closer to the actual in-game sprite from the original game.
I've always liked the official art for Dig Dug and Pooka, so it was fun to put them in that style. I just liked Fygar's in-game sprite better, especially since seeing him still irritates me even when I'm not playing the game.