Skulkers are a powerful species typically classed as mammals. Because thats what they are. These massive beasts normally get up to 12ft tall though there are some exceptions within other regions, such as the cave-dwelling skulkers that only grow up to 7ft - or the Skulkers hidden in plain sight along the rims of grasslands that can get up to 15ft.

Behaviour+things that could change it.
These creatures are unpredictable, there are few recorded cases of them actually interacting with humans; the most known cases being where Skulkers have attacked or mauled humans for seemingly no reason. They are hostile creatures but do not kill for sport, being passive around animals they see as harmless. Skulkers are pack creatures, living their whole lives in groups of up to 20 at a time. These groups are led by one male and one female, usually a pair of mates - who then lead every hunt and train most of the young. If a pack-born Skulker is separated from their pack too quickly, the creature can experience massive changes in behaviour and become much more hostile due to the increased risk of injury when left alone. Skulkers do not normally interact with other species, but when stripped from their social group a lone skulker can pack bond with practically anything.

Lifespan and predators/threats.
Skulkers typically live a bit over 100 years and are unaffected by the typical "aging". They are usually killed off by sickness or other animals before they even reach their limit, so nobody really knows how long they live. Skulkers are an INVASIVE species and are not normally welcomed by Earth animals, so they hide and keep out of sight for as long as they can. They have no purpose in the food chain, and no natural predators, as these creatures are not from Earth. Their pelts are too thick for a typical animal to bite through and even if something succeeded in killing one, skulker flesh tastes rotten and toxic. Probably isn't edible for earth animals.
Despite this Skulkers aren't invincible, they can be weakened. Whether it be by age, deformations, sickness, or just general good combat skills. Skulkers keep their distance from bulky creatures like Hippos or elephants, since its a pain to kill them and they just find it not worth their time.

Skulls.
Skulker skulls are probably one of their more defining traits. A skulker without a skull is a fly without wings. The skulls are seen as a sort of show of dominance, and the respect you get depends on what skull its from. Say a skulker had the skull of a fox. they would be bullied and experience food aggression by many more skulkers if they happen to live in a pack. If a Skulker had a rhino skull they would be significantly more respected, and perhaps even receive a better rank. Skulkers that lack a skull completely are looked down upon, as they are seen as weak. If a Skulker breaks the skull of another Skulker during a fight or tussle, both skulkers will be exiled as it is seen as a horrific act.