They are former Ninjalinos, who were transformed into werewolf-like creatures by using Sticky Splat to glue fur from the Wolfy Kids to their body. Marvin was the first Ninjalino to accidentally transform this way when Night Ninja ordered his Ninjalinos to pelt the Wolfy Kids with sticky splat, but Rip threw one sticky splat projectile back at them. The following night, the other Ninjalinos repeated the process on themselves so they could abandon Night Ninja and join the Wolfy Kids, until the PJ Masks helped Night Ninja to convince the Werejalinos they were better off as ninjas.
The Werejalinos are identical to the Ninjalinos, except that they have visible purple hair at the sides and above their heads. They act more animal-like than Ninjalinos, like running on their hands and feet.