Maybe it's something related to
guns
I almost am a chicken,
cock (male chicken, a gun part)
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,
grip (the flu, gun handle)
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)