Lava Cheep Cheep
From the Super Mario Wiki
Lava Cheep Cheeps (also described as "black Tobi Lava Fish") are Cheep Cheeps that appear in Super Mario Sunshine. Their scales are a dark mahogany color, and they are constantly on fire. They inhabit the magma within Corona Mountain, where they swim in one direction a short distance, and then leap back to where they started in an arc. None of them are even remotely in Mario's way, however.