Transformations

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. 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

Lord of The Flies

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

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

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
Brimstone

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

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

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

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!
The Poop
E. Coli

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 Bum
Key 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
Telepathy For Dummies

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

Adulthood

AFTERBIRTH+ ONLY

Isaac grows hair all over his face and gains +1 HP. Taking three puberty pills will give Isaac the adulthood transformation: