Monstro = Lil Monstro
Larry Jr./Pin/Scolex/The Frail = Leech (Afterbirth+)
Larry Jr./Hollow = Leech (Repentance)
Pin/Scolex/The Frail = Worm Friend (Repentance)
Duke of Flies/The Husk = Rotten Baby
Chub = Little Chubby
Chad = Little Chad
Carrion Queen = Big Chubby
Peep/Bloat = Peeper's Eye
Loki/Lokii = Lil Loki
Gurdy/Gurdy Jr./Mama Gurdy = Lil Gurdy
Fistula/Teratoma = Tinytoma (Repentance) (you can only have one timer at a time)
Blastocyst = Mystery Egg (Suggestion by Meman82)
Gemini = Gemini
Blighted Ovum = Lost Soul (Repentance)
Steven = Little Steven
The Fallen = Incubus
Krampus = Lil Brimstone
Headless Horseman (the body) = Headless Baby
Monstro II = Robo Baby (Afterbirth+)
Monstro II = Mongo Baby (Repentance)
Gish = Little Gish
The Widow = Juicy Sack
The Wretched = Sissy Longlegs
Daddy Long Legs = Daddy Longlegs
The Haunt = Lil Haunt
The Gate = Dry Baby!
Mega Fatty = Farting Baby
Mega Maw = Buddy in a Box (I can't figure out what to give for it)
The Gate = Dry Baby
Mega Fatty = Farting Baby
The Cage = Lil Spewer
The Dark One = Demon Baby
The Adversary = Succubus
Polycephalus = Buddy in a Box (I can't figure out what to give for it)
Mr. Fred = Harlequin Baby
Uriel = Guardian Angel
Gabriel = Sworn Protector
The Stain = Jaw Bone
The Forsaken = Ghost Baby
Mask of Infamy (mask) = Infamy
Mask of Infamy (heart) = Isaac's Heart
Rag Man = Buddy in a Box (I can't figure out what to give for it)
War/Famine/Pestilence/Death/Conquest = Little Harbingers

Dingle/Dangle/Brownie = Poo Bum (I made it :D)
Little Horn/Big Horn = Little Little Horn (I made it :D)
Hush = Little Hushy

Repentance only:
Sloth = Bob's Brain
Lust = Acid Baby
Wrath = BBF
Gluttony = Big Fan
Greed = Bum Friend
Envy = Giant Cell
Pride = Rainbow Baby
Baby Plum = Fruity Plum
Bumbino = Bumbo
Reap Creep = Intruder
Lil Blub = Leech x 2
Rainmaker = Lost Soul
Min Min = Pachal Candle
Colostomia = Sack of Sacks
Turdlet = Blood Puppy
Tuff Twins/The Shell = Bird Cage
Great Gideon = nothing (sorry)
Singe = Lil Dumpy
The Heretic = Psy Fly
The Visage = Isaac's Heart
The Horny Boys = Lil Loki x 2
Rotgut (final phase only) = Rotten Baby
Chimera = Boiled Baby
The Scourge = Pointy Rib

Clog = Poo Bum (I made it :D)
Hornfel = Little Little Horn (I made it :D)
The Siren = Little Siren (I made it with love :D)