I remember a fanfic along time ago where Harry created this magical being, like a tiny creature that ran around hogwarts and fucked with people that weren't nice to them.

 

Harry made Peeves?

 

Nah, he made a tribble that's running around scaring off Klin... Slytherins.

Here is a fic that features Harry creating this magical being, like a tiny creature that's - kind of - running around Hogwarts ...

 

Here's one where he creates an army of minor magical creatures modelled on cats.

 

Was it 'Avatar of a Thundercat', by any chance? Some shitty Harry/Hermione/Luna.

 

I seem to recall a crack fic where Harry starts making 'The Forbidden Potion' in his first year's potions class, where the recipe is the 'eye of newt and toe of frog, wool of bat and tongue of dog' potion from MacBeth. He succeeds, and manages to create a small army of Pikachu's. Can't seem to recall the title though.

 

I thought it was something to do with Thundercats too, but that one's not it. I've goggled a ton for it.

The only thing I remember is that Draco went to bother Harry on the train, Harry made his eyes turn bright green (maybe?) and gave a fake prophecy (maybe just spoke in a deeper voice?) and told Draco to be nice to the little, or be nice to the tiny people.

 

That's the one I linked to. Chapter six for the eye glowing thing, chapter seven for the thundercats.

 

Holy shit really? I guess I didn't skim far enough into it!

 

Pretty sure that was that one Deadpool crossover.

 

Thinking in little green boxes, was its name. Remember enjoying it, though I dont think its been updated in forever.