cerber

1. Croat. (in mythology) Cerberus
   Cerber je troglavi pas iz grčke mitologije. — Cerberus is a three-headed dog from Greek mythology.
2. (of a person) a stern guard; watchdog
   Portir je pravi cerber, nikoga ne pušta unutra. — The doorman is a real watchdog, he won't let anyone in.

In Serbia the mythological dog is called Kerber; in the figurative sense 'stern guard' both kerber and cerber occur, kerber being the more common.