Believe he only spawns at night. Is in the first small area northwest of Gukta in Rathe Mountains on a hill with a bunch of mosquitos/larva roaming around. Spawned for me twice in a row as soon as 7pm and the mob population in the area began to rapidly change from mosquitos/larva to trolls/frogloks. All of this could be coincidence though.
Yun Jerahk the Poisoned is REAL annoying. KOS to regular frogs, and has a habit of jumping unsuspecting newbies. "Look, Yun Jerahk...the Poisoned? Ahh!!"