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They are a sub-species of the weaker Harimen. There are a few differences between these two. Visually, they have a black shell rather than pink.
Defeating them requires different methods too. A standard ground pound is incapable of flipping them over; if Wario performs this, the Harimenzetto will be unaffected, but will walk towards Wario instead. They are dealt with the same way as Harimen; smashing their unprotected underside when they are flipped over. To flip a Harimenzetto, Wario must either perform a super ground pound, throw an enemy (or rock), or if Wario is below them, flip them over by having his head hit their underside. Note that if any enemy other than a Ringosukī is thrown at the Harimenzetto, the thrown enemy will be defeated. Throwing a Ringosukī at the Harimenzetto defeats both of them. They can also be defeated instantly by Fat, Frozen, or Zombie Wario along with rolling if a slope is provided.
When defeated, they yield three silver Coins. Wario loses a gold Coin (or equivalent if he does not have enough) if he is hit by them.
Harimenzetto are fairly rare to encounter. On Super Hard mode, most of the Harimen will be replaced by Harimenzettos.