It spits out a horse and an apple.

i feed it an ion from Titanfall 2

It spits out the ion

I feed it a gun.

It spits out a bigger gun

i feed it big sword

It spits out the olde sword. I feed it a FUTURISTIC GUN :D

It spits out a GREEN-clearance Laser Barrel. On picking it up, you are immediately terminated for unauthorized possession of higher-clearance items, and Master-O-DAB-1 is replaced by Master-O-DAB-2.

(Kudos to anyone who got the reference)

I feed the mimic a mimic action figure.

*replaces with my stuff*

Oh No you don't

*Replaces  their stuff with the mimic action figure again*

 I feed it comic sans M.S.

Glad to have an opportunity to NOT be an action figure cannibal, the mimic gratefully ingests Comic Sans.
He then becomes enlightened into the great divine power of His holy print, and spends the next 0.99999999 seconds going around the world posting "99 reasons why comic sans is awesome" on the doors of every church and Microsoft employee's cubicle. It then spits out another copy for you to read.

I give up trying to feed the Mimic an action figure, and instead feed it all three Georgias: The state, the country, and the font style. Florida is now lonely.

I feed it memes subreddit then

(Don't forget you also have to explain what happens when the person above you feeds the mimic)

The person above became a subreddit meme

It spits out confusion

i feed it the concept of mortality, and that we will never truly know what is on the other side of the veil until we reach it.

feeds it this subreddit.

Quote from Ahzehk_Ahriman >>

It spits out confusion

i feed it the concept of mortality, and that we will never truly know what is on the other side of the veil until we reach it.

Subreddit? What subreddit.

Your supposed to respond to the post above you by adding something it spits out. It kinda defeats the purpose of the game if your only writing what you feed the mimic and ignore the post above you. 

The mimic spits out air. Poor mimic wasn't fed...

I feed the mimic... helium....

The mimic floats into the sky!
Then he lands. The earth shakes. He coughs up a balloon he accidentally swallowed.

I feed the mimic a nine-course meal.

The mimic bursts.

I feed it the person above me

After returning from being busted by a 9-course meal, the poor Mimic gets a person shoved down its throat.
(Thanks for that, it's really cramped in here...)

He then spits the person back out. Human tastes disgusting, and mimics have morals too.

I feed the mimic twenty-five dollars.

The mimic eats the "green paper", and spits out some candy it bought with it.

I sprinkle the person above me with seasonings and salts then feed them to the mimic.