I replaced the turtles with snails. Since Bowser is a dragon turtle, I figured I should make a dragon snail. I started with 64x64 so that I could use it in Pixel Critters and then scale it down to 32x32 for Mari0. That was a ugly. I did make a 32x32 version which is in github. It's still ugly but can work as a temporary replacement.
I kept thinking of the Pixel Critters version so I decided to try again. I changed the angle slightly so that you aren't looking at the side. I much prefer this version.
EDIT: For those who think this Pixel Critters version still isn't very good pixel art, you are right. I know I'm not a great pixel artist. I did all the monster art for Pixel Critters so that it was homemade even if I could get better artwork from opengameart.