The spells Phantasmal Force and Major Image allow you to create the illusion of a creature, here for example the text from Major Image:
You create the image of an object, a creature, or some other visible phenomenon that is no larger than a 20-foot cube.
This explitly refers to "a" creature, in singular. How does this work with a swarm, for example the swarm of rats, which is a single monster. Does the spell allow you to create the illusion of such a swarm?