Well, a Goomba is basically an anthropomorphic mushroom. Something in the Foongas/Emoongas line?
Chestnut, actually, though that still means it'd be Grass as its base.
Its evolutions would have to branch, though; Flying/Grass type Paragoombas, Normal/Grass type Tanoombas, Fire/Grass type Jack O'Goombas, Water/Grass type Octoombas, Dark/Grass type Gloombas... and probably more I'm not thinking of.
It'd also have incredibly poor stats that make its type variations and rarer type combinations more trivial than useful, of course.
The Goomba's natural learnset includes Tackle, Bite, Poison Sting (Thornet), Headbutt, Foresight (Tattle), Stomp (the shoe), Helping Hand (Rally Wink), Double Hit (Multibonk), and Belly Drum (TTYD Charged). Its standard abilities are Run Away and Wimp Out, but it can occasionally be found with Sturdy (Yoshi's Island).