Appearance and Role in the Healthy Band
The Monster Cans appear to be huge, yellow versions of Spinach Can. They have 6 teeth, a long nose, and stubby black arms.
Seeing that they appear in Don't Hug Me I'm Scared 5 and resemble food, they are most likely affiliated in some way with The Healthy Band. They also look similar to lead singer Spinach Can. We can assume they are special members of the band who make sure nobody wakes up.