Creatures from the NegaVerse come in all shapes and sizes. There aren't any true species, as such, but the creatures from that realm do sometimes take recognizable, generally repeatable forms. Others are differentiated from their fellows by the abilities they possess.
NegaDemons are monstrosities that have somehow gained some form of Character, despite all odds to the contrary. Oftentimes, these identities will be granted unto them by NegaHumans. These Demons then serve as powerful minions for the Humans, doing their bidding by spreading Nega impurity throughout the Universe. Ostensibly, anyway, as most NegaHumans kind of suck and tend to become embroiled in petty plots and themes. The true threat, then, are the NegaDemons who spawn randomly from the pits of the NegaVerse. They are unfettered by petty conflict and their hatred of purity burns hotter than a NegaHuman's ever could. They become the leaders of the Nega Hordes.
NegaHumans are the NegaVerse's Human champions. They see worth in the NegaVerse and devote some to all of their Creativity toward fostering it. The existence of NegaHumans can be a little confusing to outsiders as they are, in fact, technically the same person as their normal Human counterpart. This is because Humans occupy all reality across Toy MultiVersal spectrums. To help make sense of this anomaly, NegaHumans are easily distinguished by their SpockBeards.
NegaVamps are the Heroes of the NegaVerse, a new breed of creature given Character through Theme. They have, over time, evolved the ability to suck out and absorb the pure ABS found in minifigs and similar Leg-Ore evolved creatures. They incorporate this ABS into their own bastardized genetik makeup, becoming stronger, faster, smarter, more well-built, and more articulate versions of themselves. In short, living nightmares for BrikVersians. NegaVampirism is an aggressive disease and it has begun to spread throughout the populations of the NegaVerse. If it is not stopped, soon all the Nega denizens will be NegaVamps.
POOP (Pieces with Only One Purpose) Creatures are the lowest of the low. They are solid, one part conglomerations of fused ABS. They are frighteningly effective at whatever it is they were made for or for whatever purpose to which they evolved, but they are distinctly lacking in versatility. Humans find them distasteful as they do not satiate Creativity or ingenuity. They feel that POOPs remove choice. NegaHumans find them delightful in that POOPs allow them to focus their Creative energies on more important brikbuilding tasks. Humans call NegaHumans lazy for this reason and NegaHumans respond by saying that their dragons look way cooler. Chessmen, technically POOPs themselves despite not being Nega (the term merely originated in the NegaVerse), find BrikVersian Snobbishness in this regard to be a particularly delicious irony.