The phrase encompasses great big outfits padded to give an unhuman shape - such as
the performers/actors you might expect to see wondering about in Disney Land and also much smaller scale costumes. The smaller scale outfits are sometimes known as 'disguise pyjamas' or 'animal pyjamas' . These are light-weight all-in-one jumpsuits, with a hood to give the wearer a new identity. As long as a costume looks cartoony or stuffed-toy like, it pretty much counts.
In Japan, kigurumi animal pyjamas have become fashionable streetwear, and it is not unusual to see the latest manga characters or a tiger walking down the streets of Tokyo late at night. Or early in the morning come to think of it. These costumed idivduals are known as 'kigurumin'.
In the west a similar craze exists, known as fursuits, furplay or furries. Although this has yet to venture out from close-knit conventions and private parties into the realms of street wear, largely due to the prevalance of the Disney-style padded costumes. Difficult to negotiate narrow doorways, you might imagine.
However, the lighter wight kigurumi animal pyjamas have started toemerge in the West in 2010. Lilly Allen famously sported a Dinosaur at Bestival in 2009. The 2010 UK Festival circuit has seen an explosion of these costumes and it looks like they are set to stay. For example check out the selection at the Animal Costumes Shop