From the Super Mario Wiki, the Mario encyclopedia
Goomba Troopa are a species in Monster Mix-Up. They are the result of mixing Goomba and a Koopa Troopa, creating a single being that has the Koopa Troopa's head with the Goomba's body. After Mario, Toad, and the Mushroom King stumble across an army of mixed Koopa Troop enemies, a Goomba Troopa notices them and warns the leaders, Goomby and Foomby (which are also Goomba Troopas) about the heroes. Mario recognizes the leaders and promptly kidnaps them, although he accidentally kidnaps a pair of Goombombs as well. As Goomba Troopas and Goombombs are identical to each other, Mario cannot tell which is which unless the reader solves a puzzle. By solving the puzzle correctly, Mario lets the Goombombs blow up and knocks out Goomby and Foomby, although the Mushroom King is then kidnapped by the rest of the army.