Gives invulnerability and +0.28 speed for six seconds, removing the ability to shoot tears and replacing it with the ability to deal contact damage to enemies.
Isaac deals 40 contact damage to enemies, once per second.
Isaac deals 20 contact damage to enemies, twice per second.
If an enemy is at 40 HP or less, the contact damage is increased to 40 for that enemy.
Ending the current run via opening the end chest while My Little Unicorn is active will cause the credits and main menu music to be sped up. The effect ends upon restarting the game or starting a new run.
VII - The Chariot card provides the same effect of this item when used.
Synergies
4.5 Volt: With some damage source while this item is active, it can be permanently invulnerable inside the room while you have enemies.
If combined with 9 Volt, it will be easier to achieve invincibility.
???'s Only Friend: The fly can still be controlled while invincible.
Book of Virtues: Spawns rainbow-colored wisps in the middle ring. The wisps orbit Isaac more quickly than normal wisps and change colors constantly, with both of these effects being boosted while My Little Unicorn is active. The wisps also deal an extra 8 contact damage, for a total of 15 contact damage per tick. When de-activated, the rainbow coloring slightly fades away.
Car Battery: Doubles the duration of invulnerability.
Mom's Knife does contact damage in the same direction it was being held at the time of using it, giving a massive amount of free damage by ramming into an enemy.
If you have the Yes Mother? Transformation, you can basically do the same thing using this item.
3DS The icon of My Little Unicorn as seen on the lower 3DS screen.
Trivia
The item is a reference to the show My Little Pony: Friendship is Magic. The unicorn is one of the three types of ponies in the show, the others being the pegasus and the earth pony. A notable unicorn from the show is Twilight Sparkle, its main character.
In the original Binding Of Isaac, using the item changed the music instead of speeding it up.
If you pause the game while under the effect of My Little Unicorn, the music plays indefinitely.
Isaac deals 40 contact damage to enemies, once per second.
Isaac deals 20 contact damage to enemies, twice per second.
Blood Donation Machines and 
Devil Beggars.
VII - The Chariot card provides the same effect of this item when used.
4.5 Volt: With some damage source while this item is active, it can be permanently invulnerable inside the room while you have enemies.
9 Volt, it will be easier to achieve invincibility.
???'s Only Friend: The fly can still be controlled while invincible.
Book of Virtues: Spawns rainbow-colored wisps in the middle ring. The wisps orbit Isaac more quickly than normal wisps and change colors constantly, with both of these effects being boosted while My Little Unicorn is active. The wisps also deal an extra 8 contact damage, for a total of 15 contact damage per tick. When de-activated, the rainbow coloring slightly fades away.
Car Battery: Doubles the duration of invulnerability.
Mom's Knife does contact damage in the same direction it was being held at the time of using it, giving a massive amount of free damage by ramming into an enemy.
Aries, 
The Virus, etc.) and Orbital Familiars (
Cube of Meat, 
Sacrificial Dagger, etc.): Deals damage on top of My Little Unicorn's damage and invincibility.
The Binding of Isaac: Rebirth
