From the Super Mario Wiki
Fangs are blue bats with red wings that have the same mannerisms as the Swoopers. Fangs appear in the cave levels of Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Super Princess Peach, Yoshi's Island DS and Yoshi's New Island. When Yoshi (or the protagonist of whatever game they are encountered in) gets too close to one, the Fang will fly in his direction from its sleeping position. For some reason, when Fangs fly, small, white dots appear behind them before disappearing. Yoshi can lick one up to make an Egg.
Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Fangs appeared in Super Mario World 2: Yoshi's Island as enemies to Yoshi which tried to damage him by swooping down at him in all of the different cave levels. The Fangs will attack Yoshi if Yoshi gets too close to them because that is the action which triggers their flight.
Yoshi's Island DS
In Yoshi's Island DS, Fangs came back, and were classified as Airborne Enemies (proven by the Island Museum). Fangs were seen more in cave areas than in airborne areas, though. Their first appearance in that game was in World 3-5, named Heeeeeeere's Wario!
Yoshi Touch & Go
Fangs were also seen in Yoshi Touch & Go, where they patrolled ground areas and appeared not only in caves, but also in additional areas such as jungles and fields. These Fangs could only be found sleeping on ceilings in cave stages, and were otherwise always in flight, soaring up and down at varying intervals. If shot down with an egg or stomped, these Bats were worth two points, and encasing them in a bubble could earn the player a Blue Coin. On a related note, Bats were also present in the Balloon Trip tech demo which eventually evolved into the final Yoshi Touch & Go. In Balloon Trip, Fangs were found in the sky (an area they were ironically absent from in Yoshi Touch & Go) and continually circled around; such a role was taken up by red Fly Guys in the final Yoshi Touch & Go.
Yoshi's New Island
Yoshi's Woolly World
Fangs reappear in Yoshi's Woolly World made out of wool. When the fangs fly they leave long, purple dashed trails.
Super Princess Peach
Fangs have later appeared in the Nintendo DS game, Super Princess Peach. Once again they are enemies which try to defeat the protagonist, but this time it is Princess Peach instead of Yoshi. They appear along with another variant called Glad Fangs and fly right at Peach instead of just around the screen. In several levels they also create walls which Peach must remove by using the Rage vibe and burning them apart.
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