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

I almost am a chicken,
>! cock (male chicken, a gun part)

Not quite a famous horse.
>! [Trigger](https://en.m.wikipedia.org/wiki/Trigger_%28horse%29)

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
>! draw (draw with a pencil, draw a gun)

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