Sith beastmasters were the members of the Sith Order

Sith Order Organization Reconstituted Sith Empire

of the reconstituted Sith Empire during the Great Galactic War

Great Galactic War Event Great Galactic War (3681BBY - 3653BBY)

, Cold War and beyond. Their primary specialization was to subdue the will and then to train various beasts and creatures, and to command them in battle - or to transfer the control of the trained beasts to non-Force users. They were experienced Force-wielders and were proficient in a lightsaber combat.
On Balmorra

Balmorra Planet

, the beastmasters were in charge of training Sith hounds

Sith Hound Creature Non-sentient

and commanding them to patrol the road to a nearby Imperial Command Compound along with Imperial troopers

Imperial Army (Sith Empire) Organization Reconstituted Sith Empire

.