Geronimouse is the Chief of the Mousehican tribe in An American Tail: Fievel Goes West, situated in the "Worselands", a desert in the west. He is their spiritual leader. After they capture Tiger, he is the first member of the tribe who notices Tiger's resemblance to their sacred rock, which causes the Mousehicans to worship him as a god instead of eating him. He is implied to have shamanic magical powers, like summoning fire.
Geronimouse is named after Geronimo, a prominent leader of the Apache tribe whose name means "One who yawns". Similarly, the Mousehicans are named after the Mohican Native American tribe, who did not actually live out west but in the New England area.
While a minor character, he was given a card in the An American Tail Trading Cards. This is the only time he is named.