There are 3 Primeval Bears in the Wyvern Mountains, but after killing them, they don't respawn another time.
Primeval Bears for grinding dwell in the Bloodpaw Cave (Nilomar Cape area on the global map). Usually there 3 beings - one in a secret room (rumor bosses spawn place, just in case) and other two always in the separate rooms of the cave.
Btw, Primeval Mantle is good for selling (the whole 1500), but only if you already have for yourself, because it's hard to obtain (not hard as damn Devourer Sting).
For fighting Primeval Bears on low levels it's recommended to use some good fire or anti-beast weapon (but not necessarily) and have some cold resistance.
(This info mostly for new players for the future).