Ah, resolved my issue with my theory, so now it's a guess.

I almost am a chicken,

X = (Eggs)

Not quite a famous horse.

C = (Sea)horse

Confined to my domain,

N = (in)

I'll help you see the course.

I = (eye)

I almost am contagious,

?

Not quite a Youtube part.

Q = (queue)

Use a pencil when you're ready

?

And light me when you fart.

: = (colon) You are a keyboard.

Ah, resolved my issue with my theory, so now it's a guess.

I almost am a chicken,

X = (Eggs)

Not quite a famous horse.

C = (Sea)horse

Confined to my domain,

N = (in)

I'll help you see the course.

I = (eye)

I almost am contagious,

6 = (sick)

Not quite a Youtube part.

Q = (queue)

Use a pencil when you're ready

Right-arrow = (write)

And light me when you fart.

: = (colon) You are a keyboard.

So, I had a fun little theory running, and if I weren't so sure the last line refers to

Incense/Candle

I'd pursue it:

I almost am a chicken,

X = (Eggs)

Not quite a famous horse.

C = (Sea)horse

Confined to my domain,

N = (in)

I'll help you see the course.

I = (eye)

I almost am contagious,

?

Not quite a Youtube part.

Q = (queue)

Use a pencil when you're ready

?

And light me when you fart.

?

Ah, resolved my issue with my theory, so now it's a guess.

I almost am a chicken,

X = (Eggs)

Not quite a famous horse.

C = (Sea)horse

Confined to my domain,

N = (in)

I'll help you see the course.

I = (eye)

I almost am contagious,

?

Not quite a Youtube part.

Q = (queue)

Use a pencil when you're ready

?

And light me when you fart.

: = (colon) You are a keyboard.