Partial... Maybe it's something related to
>! guns?

I almost am a chicken,
>! ?

Not quite a famous horse.
>! ?

Confined to my domain,
>! range (a function's range, shooting range)

I'll help you see the course.
>! sight (eyesight, guns can have sights)

I almost am contagious,
>! ?

Not quite a Youtube part.
>! clip (videoclip, clips of ammo)

Use a pencil when you're ready
>! ?

And light me when you fart.
>! fire (lighting a match, gunfire)