The clavotrapezius is found craniodorsal on the cat and makes up the cranial portion of the brachialis muscle. It is viewed as number 1 on this photo. This muscle pulls the limb forward, extends the shoulder, and depresses and pulls the head and neck laterally.
HINT: the clavobrachialis is also visible here. Scroll down to see the number it is labeled.

Clavobrachialis: 4