From the Super Mario Wiki
The title of this article is conjectural; an official name for the article's subject has not been found, so it has been given a fitting title by editors. If an official name appears, it is requested that the article be moved to the correct name.
A Winged Anvil.
The Winged Anvil is an enemy found in Yoshi's Safari during the boss fight against Wendy O. Koopa. They look like anvils with wings, and flutter back and forth over the pipes Wendy pops out of to throw Bombs at Mario and Yoshi. The Koopaling is shielded from the blasts fired by Mario's Super Scope, so the only way to defeat her is by shooting an anvil out of the sky and onto her head. This can only be done when the anvil faces Mario in preparation for its own attack (shooting horseshoes as the protagonists), since its sides are impervious to the Super Scope, while its face is not. The anvils do not always pause over Wendy's head, and while there is an unlimited amount of anvils, it takes time for a defeated enemy to be replaced by a new anvil (there is only one on screen at a time), so it is wiser to wait until Wendy and the anvil are lined up before shooting.
The horseshoes fired by the anvils can be destroyed using shots from the Super Scope.