Did you know that in The Binding of Isaac: Rebirth Is it possible to transform into some special creatures by collecting specific items? Whether the answer is yes or no, a nice guide to know how to get them all is always handy.
The transformations were originally two, but with the DLC Afterbirth they became eleven in total. They grant unique powers and are activated when three items from their item set have been collected.
Here is a list of all the transformations and their sets:
How to activate all transformations in The Binding of Isaac: Rebirth and Afterbirth
Rebirth
guppy: 
- It will allow you to fly
- Every time a tear hits an enemy, it will create an ally blue fly
Set:
Guppy's Head
Dead Cat
Guppy's Collar
Guppy's Paw
Guppy's Tail
Guppy's Hair Ball
Beelzebub:
- It will allow you to fly
- The small enemy red flies will become allied blue flies
Set:
??? ”s Only Friend
BBF
Best Bud
Big Fan
Distant Admiration
Forever Alone
Halo Of Flies
Hive mind
Katole
Smart Fly
The Mulligan
Infestation
Friend zone
Lost Fly
Papa Fly
Obsessed Fan
Afterbirth
Bob:
- You will leave a trail of poison behind you, damaging and poisoning the enemies who step on it
Set:
Bob's Rotten Head
Bob's Brain
Bob's Curse
Ipecac
Conjoined:
- You will also shoot diagonally as well as in a straight line
- -0.3 Damage
- Slightly decreased rate of fire
Set:
Brother Bobby
Harlequin Baby
Headless Baby
Little Steven
Mongo Baby
Rotten Baby
Sister Maggy
Fun guy:
- +1 Health Up
Set:
God's Flesh
Mini Mush
Magic Mushroom
Odd Mushroom (Thin)
Odd Mushroom (Large)
1up!
Blue Cap
Leviathan:
- It will allow you to fly
- +2 Black Hearts
Set:
The nail
The Mark
The Pact
Pentagram
Spirit of the Night
Maw of the Void
Abbaddon
Brimstone
Oh Crap:
- You will cure yourself of half a heart by destroying the feces
Set:
Flush!
The Poop
E coli
Seraphim:
- It will allow you to fly
- +3 Soul Hearts
Set:
The Bible
Dead Dove
Guardian Angel
Holy Grall
Holy Mantle
Mitre
Rosary
Sworn Protector
The Halo
spun:
- +2 Damage
- +0.15 Speed
- A pill at the time of transformation
Set:
Experimental Treatment
Growth Hormones
Roid Rage
Speed Ball
Synthoil
The Virus
Yes, Mother?:
- A knife will appear behind you, you can move it by moving and it will do damage on contact like the Mom's Knife
Set:
Mom's Bottle of Pills
Mom's Bra
Mom's Pad
Mom's Wig
Mom's Eye
Mom's Purse
Mom's Contacts
Mom's Knife
Mom's Heels
Mom's Eyeshadow
Mom's Perfume
Mom's Key
Mom's Lipstick
Mom's Underwear
Mom's Coin Purse
Mom's Pearls
Super Bum:
- Combine the three pets needed for transformation to create Super Bums. He will do all the duties of the three, but with double rewards
Set:
Bum Friend
Dark Bum
Key Bum
