Bats
Bats are flying enemies which typically chase Isaac, and can attack by making quick lunges or, in the case of fatter variants, by spitting projectiles.
This page is about Bats added by Fiend Folio. More information about Bats in general can be found in the vanilla page.
Milk Tooth
Milk Tooths are juvenile 
One Tooths that behave similarly. They receive increased knockback from Isaac's tears, especially while charging, and will take time to accelerate and decelerate when charging.
Foamy
Foamies appear to be infected One Tooths. Instead of flying after Isaac, they drift through the room in random directions while twitching. After a while, they snarl and charge rapidly after Isaac in a straight line, bouncing 3 times off of walls before stopping. Whenever they bounce, they release blood and bone projectiles in random directions.
Bubble Bat
Bubble Bats are a drowned variant of 
Fat Bats that have similar behavior, attempting to align themselves diagonally and will only attack if Isaac is diagonal from them. Instead of a normal projectile, Bubble Bats will fire a spray of randomly-sized 
Bubbles towards Isaac.
Upon death, Bubble Bats will fall down, crushing any obstacles below and launching a small shower of projectiles.
Jawbone
Jawbones are skeletal One Teeth with less than half their health, but otherwise behave identically. Upon death, they spawn a skull, referred to as a Jawbone Corpse, that can be revived by Ribbones.
Jawbone corpses will block Isaac's tears, impede his movement, and will be immediately destroyed once all Ribbones are killed.
Jawbone corpses can also occasionally spawn on their own, but will always be accompanied by Ribbones.
Ribbone
Ribbones are skeletal Fat Bats that very slowly move towards Isaac. However, if a Jawbone Corpse is in the room, they will begin moving quickly towards it and will revive the Jawbone upon reaching it. They can revive multiple Jawbones at once if the skulls are grouped together.
Trivia
- In the MOD's files, Milk Tooth is referred to as "Baby Tooth".
- Ribbones revive Jawbones with red-colored magical beams in Necropolis and purple-colored beams in Mausoleum.
- Ribbone's glowing eye when they're about to revive Jawbones is likely a reference to the character Sans from Undertale.
Gallery
A charging Milk Tooth.
A charging Foamy.
A Jawbone Corpse.
Jawbone's appearance in Mausoleum.
A Ribbone's appearance when reviving Jawbones.
Ribbone's appearance in Mausoleum.
Old appearance of Foamy in Afterbirth+.
Old version of Ribbone.
