"You've met other moles while wandering in the caves, but you've never seen one this big or brutal. Use Psi power to both attack and heal, and try to keep Paula in the pink until the end of the skirmish. It might take a couple tries to topple him."
EarthBound Player's Guide

Mondo Mole is a boss in EarthBound that guards the second Your Sanctuary location, Lilliput Steps. It appears to be an oversized Mole Playing Rough or alternately colored Guardian Digger, with what may be blood trickling down its mouth and claws. It can bash, tear into, or claw at a single character, or it can use the α level of Lifeup, PSI Shield, and Offense up.

Defeating Mondo Mole is not required to advance the story to Threed, however you must eventually defeat it to gain access to Lilliput Steps and the second melody.