From the Super Mario Wiki
Mace Guys are mace-swinging Shy Guy who appears in only three levels (Sluggy The Unshaven's Fort, Raphael The Raven's Castle and Ultimate Castle Challenge) in the game Super Mario World 2: Yoshi's Island, as well as an additional secret level (Endless World of Yoshis) in Yoshi's Island: Super Mario Advance 3. The mace can hit a Yoshi and hurt him. A Yoshi can also eat the mace and the Mace Guy becomes a normal Shy Guy.
Yoshi Topsy-Turvy has Mace Guys in 7 missions. All but one are in Chapter Six, and a lone one is in Chapter Five.
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