The Binding of Isaac Rebirth introduced a couple of transformations to the game, but the Afterbirth DLC comes packed with an additional 9 to play with and Afterbirth+ introduces 3 more.
A transformation gives you special abilities in addition to your current form and each one holds a unique effect to help you along your way. The criteria for each one is listed below and they all involve picking up a specific combination of items.
Guppy
Isaac turns into his cat Guppy and gains the ability to fly. Every time you successfully hit with a tear, a blue fly will spawn (In Repentance this was nerfed to 50% of tears that hit). Taking any Three of the following items will trigger the Guppy transformation.
Guppy's Collar
Dead Cat
Guppy's Hairball
Guppy's Head
Guppy's Paw
Guppy's Tail
Guppy's Eye
Kid's Drawing
Beelzebub
Isaac turns into a human fly and gains the ability to fly. This transformation will turn most types of flies into friendly blue attack flies. Taking any Three of the following items will trigger the Lord of the Flies transformation.
???'s Only Friend
BBF
Best Bud
Big Fan
Distant Admiration
Forever Alone
Halo of Flies
Hive Mind
Skatole
Smart Fly
Mulligan
Infestation
Papa Fly
Friend Zone
Obsessed Fan
Lost Fly
Jar of Flies
Angry Fly
Plum Flute
Infest
Psy Fly
Fruity Plum
Bot Fly
YO LISTEN!
Parasitoid
Mom
Isaac turns into his mother, giving him a knife which is attached to his butt and cannot be fired. Taking any Three items in this list will give Isaac the Mom transformation.
Mom's Bottle of Pills
Mom's Bra
Mom's Coin Purse
Mom's Contacts
Mom's Eye
Mom's Eyeshadow
Mom's Heels
Mom's Key
Mom's Knife
Mom's Lipstick
Mom's Pad
Mom's Purfume
Mom's Purse
Mom's Wig
Mom's Underwear
Mom's Pearls
Mom's Razor
Mom's Shovel
Mom's Bracelet
Leviathan
Isaac turns into a tentacled monster, gaining 2 black hearts and the ability to fly. Taking any Three items in this list will give Isaac the Leviathan transformation.
The Nail
The Mark
The Pact
Pentagram
Spirit of the Night
Maw of the Void
Abaddon
Eye of the Occult
Lord of the Pit
Sulfur
Seraphim
Isaac turns into a faceless angel, gaining 3 soul hearts and permanent flight. Taking any Three items in this list will give Isaac the Seraphim transformation.
Dead Dove
Sworn Protector
The Bible
Guardian Angel
Holy Grail
Holy Mantle
Mitre
Rosary
The Halo
Celtic Cross
Godhead
Sacred Heart
Holy Light
Divine Intervention
Immaculate Heart
Revelation
Salvation
Bob
Isaac turns into Bob (Sloth) and will now leave a trail of green poisonous creep as he walks. Taking any Three items in this list will give Isaac the Bob transformation.
Bob's Rotten Head
Bob's Brain
Bob's Curse
Ipecac
Fun Guy
Isaac turns into a living Mushroom and gains +1 HP. Taking any Three items in this list will give Isaac the Mushroom transformation.
Mini Mush
Magic Mushroom
Odd Mushroom (Thin)
Odd Mushroom (Thick)
1UP!
Blue Cap
God's Flesh
Mega Mush
Wavy Cap
Conjoined
Isaac grows two tumors on his face, which will fire diagonally in addition to Isaac's normal tears. This transformation is also a -0.3 damage down and -0.3 tears down.
Taking any Three items in this list will give Isaac the Conjoined transformation.
Brother Bobby
Harlequin Baby
Headless Baby
Little Steven
Mongo Baby
Rotten Baby
Sister Maggy
Abel
Demon Baby
Dry Baby
Ghost Baby
Guardian Angel
Lil Brimstone
Rainbow Baby
Robo-Baby
Robo-Baby 2.0
Farting Baby
Incubus
Multidimensional Baby
Seraphim
Acid Baby
Buddy in a Box
King Baby
Boiled Baby
Cube Baby
Freezer Baby
Lil Abaddon
Quints
Twisted Pair
Spun
Isaac turns into a buff berserker gaining +2 Damage, +0.15 Speed and spawns a random pill on the floor. Taking any Three items in this list will give Isaac the Drugs transformation.
The Virus
Roid Rage
Speed Ball
Growth Hormones
Synthoil
Experimental Treatment
Adrenaline
Euthanasia
Oh Crap
Isaac turns into a giant walking turd and he can now heal half a red heart each time a pile of poop is destroyed. Taking any Three items in this list will give Isaac the Poop transformation.
FLUSH!
Skatole
The Poop
E. Coli
Brown Nugget
Butt Bombs
No. 2
Hallowed Ground
Dirty Mind
Montezuma's Revenge
Super Bum
Picking up all 3 bum familiars will combine them and transform into SUPER BUM. Super Bum will pick up any of the other bum's pickups and drop double rewards. The Three items required are:
Bum Friend
Dark BumKey Bum
Bookworm
AFTERBIRTH+ ONLY
Isaac gains a monocle and sometimes shoots 2 shots at once, similar to the 20/20 effect. Picking up any Three book items in this list will give Isaac the bookworm transformation:
The Bible
Book of Belial
The Necronomicon
Book of Shadows
Anarchist Cookbook
Book of Revelations
The Book of Sin
Monster Manual
Book of Secrets
How To Jump
Book of the Dead
Telepathy For Dummies
Satanic Bible
Book of Virtues
Lemegeton
Spider Baby
AFTERBIRTH+ ONLY
Isaac gains blue friendly attack spiders every time he takes damage and gains a permanent familiar spiderbro, which applies a random status effect to any enemies that touch it. Picking up any Three spider-related items in this list will give Isaac the spider baby transformation:
Spider Bite
Mutant Spider
Spider Butt
Spiderbaby
Box of Spiders
Spider Mod
Daddy Longlegs
Hive Mind
Infestation 2
Juicy Sack
Sissy Longlegs
Bursting Sack
Sticky Bombs
Parasitoid
Keeper's Kin
The Intruder
Adulthood
AFTERBIRTH+ ONLY
Isaac grows hair all over his face and gains +1 HP. Taking three puberty pills will give Isaac the adulthood transformation:
Stompy
AFTERBIRTH+ ONLY
Isaac creates dust clouds when walking and has a chance to create rock waves around him after taking damage, which will spread outwards and damage enemies. (In Repentance this was buffed, you also can walk over rocks and destroy them) Taking three items or pills that increase Isaac's size will give Isaac the Stompy transformation.
Magic Mushroom
Leo
