Slowly, he came back to consciousness. Slowly, he groaned. Slowly, he blinked. Slowly, he focused his vision

alright, you're probably going to disagree with me when i say this. I understand what you were trying for here, but repeating 'slowly' makes the line monotonous and awkward. You'd be better off without it.

This realization struck him as quickly as his adrenaline shot him off the ground

this is a little confusing. Try something like "This realization struck him quick and hard, and a jolt of adrenaline shot him to his feet."

Leo’s poor adrenaline

you might want to flip that around to read "Poor Leo's adrenaline". it sounds better that way. Now Leo becomes the topic instead of the adrenaline.

Other than these flaws, the story was pretty good. Great imagery on the scenery, but i think you can make it longer. Play a little more on the details, and give us more insight on what exactly is going on. Sorry if it sounds like im bashing the piece. keep up the good work.

I liked the way you jumped straight into the action, and I liked the ending too. It makes me want to read on. =)

I agree completely with Joseph’s comment about the use of the word “slowly”— it’s very repetitive. It seems almost silly.

Turning his head to the point that he was nearly snapping his neck, something was familiar about the area.

This is a run-on. Maybe “He turned his head so far his neck almost snapped. Something seemed familiar about the area.” (I dunno; not the best -_-;; The first part of your sentence strikes me as a little awkward.)

The rest was barren dust and devoid of all life

Is “the rest” literally dust, or did you mean something like “The rest was barren; dusty and devoid of all life”?

What you must know about this person is that he epitomizes the ‘flight’ clause of the ‘fight or flight hormone’.

Show, don’t tell please. =) We don’t need to be told flat out that Leo is a chicken; just showing him running away screaming in terror is enough, and far more interesting. Also, I don’t know if you are intending to break the fourth wall. But unless the story is a diary entry or supposed to seem like a letter to the reader, I wouldn’t suggest coming right out and addressing the reader as “you”.

‘At least’ or ‘at last’, until all was painted in black. …Or was it cerulean?

I don’t understand this sentence. I’m assuming the “cerulean” part will be cleared up in the next chapter, but what are you trying to say with “at least” or “at last”?

I like the characterization of Leo so far; he should be interesting to read about if he’s so hysterical all the time. =D Keep writing!  

Mm, the others are right. It's a bit short. But from what I've seen of your other works, that's the way your process starts. You should try lengthening your pieces a little more, into the form of chapters. That way the readers will get a better feel of what's going on, I think.

I agree that the word 'Slowly' is a bit repetitive, and understand the point you were trying to get across. On the other hand, there's synonyms, or ways to get around repeating things - 

Slowly, he came back to consciousness. Slowly, he groaned. Slowly, he blinked. Slowly, he focused his vision

Why not, "Slowly, he groaned as his consciousness returned. He blinked a few times, his eyelids heavy, and he tried to focus." ... ? Just throwing it in there.

'How could this geography be? It’s impossible.'

Leo's thought process doesn't seem natural enough ... would you honestly think that way, or would you think to yourself something more along the lines of "This kind of place seems a bit impossible..." or, "Where the heck am I? This doesn't make sense."  And then describe how the landscape seems strange ... I dunno, the word geography seems a bit misplaced.

But anyway, as I said earlier, I am very curious to know what this beast is/was ... why it was there, and etc. So far it's looking good. The plot you described to me is very promising, so chop chop, make it happen! *grins*