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)