They're undead girls that behave like the animal they're dressed as. Boggleboes dressed as bears act like bears, for example, but more like how kids pretend to be animals.
A monster girl created when the soul of a deceased girl is brought back from the dead by possessing either a random plush toy or, more commonly, their favourite plush toy. The latter’s greater commonality is due to parents treasuring the deceased’s belongings so much that the chances of the soul returning to possess her plush toy skyrocket. So hey, that’s a pretty good thing about mamono! Of course, they’re still monster girls, so side effects are to be expected.
Behaviourally, they begin to exhibit animalistic traits related to the animal their plush toy is, which, as ArkhamShark said, are closer to the perception of the animal rather than the actual animal, like eating carrots as a rabbit and waving your arms about yelling gwaor as a bear. They also, more pressingly, exhibit the true physical qualities of their animals. Bear BBs possess freakish strength, rabbit BBS have keen hearing & agility, dog BBs have incredible olfactory abilities, etc. etc.
The scariest thing about Boggleboes, however, is how unassuming they are. If you were to take their costume off, they’d look exactly like they did before they became one, with no hint of mamono mana or other sort of external indicator. The only hint to their true nature would be their insistent animal play. This transfers over to their conquests as well. After their animalistic instincts prime them towards a man they fancy, they’ll try to entice him with that same animal play. To everyone else, it’s just some cutesy activity on the part of the girl. To her, it is a very real invitation. Eventually, when said invitations are not acted upon, they pounce and take the man by force. Again, to everyone else it’s just some adorable tussle between a girl and the guy she likes. For her and the man, it is a very serious struggle.
Also they build up pheromones in their suits and bomb men with them to make them lose reason, but it’s not as prominent as their other behaviour
