The Lonely Koopa Troopa
From the Super Mario Wiki
Captain Goomba, Captain Shy Guy, and Captain Boo encounter Troopeas, which immediately battle them. However, the Goomba Squad instead battles Koopa Troopas, Beanerang Bros, Limbo Bros, Fire Stalking Piranha Plants, and Koopa Paratroopas. At the end, they find a lonely Koopa Troopa who likes speaking in rhymes who immediately joins as another captain.