In Mesoamerican mythology, werejaguars were worshipped as gods by the ancient Aztecs. They referred to them as the "Nagual".
The single werejaguar seen is Kate Argent. In Season 1, Kate was killed by Peter Hale, then an alpha, ripping out her throat with his claws. The transformation took place, and Kate's body began to heal, then she revived as a new werejaguar instead of a werewolf presumably because of her sadistic personality.
In Season 5, Hayden Romero is changed into a chimera with traits from both a werejaguar and a wolf. She was drawn to and able to sense another Chimera with berserker traits. Hayden loses her jaguar traits after she is bitten by Scott McCall, becoming a full werewolf.
A werejaguar can be a former human transformed using an alpha's claws, fangs, or other means. Like other shapeshifters, they can presumably be born as well.
Their eyes glow green.
A werejaguar shares a supernatural connection with the supernatural demons, the berserkers, being able to track and sense them. They are also able to enslave and command a berserkers to do their bidding.
A werejaguars transformations can come in any combination, with practice. The enhanced senses are more permanent than the physical shifts in morphology, but even those can be controlled to some extent. Scott often has to concentrate to hear something at great distance, for instance.
The various transformations can be triggered by strong emotions, like anger, and a rapid heart rate.
Kate Argent, newly transformed as a nagual, acquired two Berserkers. They naturally came to serve her, helped her survive, did her bidding. She used them as bodyguards and man power. Kate could issue direct commands to them by roaring. When a berserker was about to kill Chris Argent, she gave out a roar and the demon spared her brother's life.
Kate used a captive Scott McCall as a human vessel for a new Berserker. She did this by the following ritual: she outfitted Scott with an animal bear's skull, while apparently falling into a trance. Scott then underwent a transformation, the Animal spirit of the demon being summoned, possessing Scott, taking over the personality of its human vessel. Afterwards, Scott was completely submissive and loyal to Kate, following her order to attack and attempt to kill his friends.
The werejaguar is a sufficient castor of magic.
The werejaguar of myth, the Nagual-jaguar god, Tezcatlipoca was worshipped at such a location by the ancient Aztecs, La Iglesia. Though Kate Argent was a newly transformed werejaguar, she was drawn to the temple of La iglesia by instinct. It was there she discovered and found two Berserkers, which became her servants due to her status as a Nagual.
The nagual-spell cast upon Derek was broken, wore off when Derek threw himself into battle to protect Scott's Pack from Berserkers, shapeshifting to his werewolf shape; he returned to his natural age and mental growth. The nagual spell also served to trigger a Wolf Evolution in Derek.