One Tooth

One Tooths are little bats that fly. One Tooths will randomly charge the player, dealing damage on contact. When not charging, One Tooths will slowly move towards the player. At the same time, One Tooths tend to gently move away if the player moves directly towards them. However, this is not advised as One Tooths usually dash when they are near the player.

Fat Bat
The Fat Bat is an enemy added in Afterbirth.

The Fat Bat moves slower than a normal One Tooth but has more health. Fat Bats try to align themselves diagonally to Isaac, giving them a tendency to avoid his shots. They will try to shoot at Isaac once they are diagonally aligned.

Trivia

 * Fat Bats were first revealed for the Afterbirth DLC in a Binding of Isaac official website post.