Bat

Bats are flying, nocturnal mammals that live on Mobius and Planet Earth, with several species found in the United Kingdom. They are often characterised as being blind, although they actually just use echolocation to "hear" their surroundings. Bats are sometimes confused with birds as they one of the only flying mammals.

Rouge the Bat is perhaps the most famous bat in the combined Sonic universes. The wily criminal was first seen in Sonic the Comic as part of The Syndicate, a terrorist organisation committed to destroying Mobius. She was actually just placed there as a Drakon spy by Master Scholar, who later withdrew her to cause several acts of terrorism on the planet. She has made a dangerous enemy of Dunzi and the Household Keepers.

Bat Brain is a Badnik obviously designed to look like a bat. Unlike real bats, Bat Brains don't need echolocation and can fire projectiles from either their mouth or tail. There have been two major designs of this Badnik, one of them containing flickies as an organic battery.

Bats sometimes appear in the Decap Attack universe. Kevin the Bat was the first character seen in any strip and was accompanied by his friends who fought Igor in vengeance. Others have appeared, particularly one that could change into a vampire.

A bat was released from inside a Buzzer in Sonic the Human! but had no dialogue.

Real, non-anthropomorphic bats have been witnessed in several Knuckles the Echidna stories as inhabitants of the Floating Island. Some of them were used as traps on the Hell House Ride and they appeared much later to scare Knuckles into almost falling off the island.

Bats were an opponent for Juggernaut in the Midnight Graveyard during Kid Chameleon story Back to Unreality!.
Bats are known residents of the Nameless Zone, two named ones being Sergeant Claw and Captain Tooth, who kidnapped resident bully Witless Chris on the fateful day Miles "Tails" Prower learned he could fly.
Tyson Dense, the mad polluter, is another example, though has only been seen on two occasions.