In order to send a guar home, a harness is required. The harnesses are found on the dead guar and used on the live guar. The Vestige do not need to collect all of the harnesses first. One is needed in the inventory in order to send it home. Once used, the harness counter will decrement in the quest tracker.
The herd of guar at the sulfur pools stampeded during the Covenant attack. Now they're scattered. If I can calm them and fit them with the proper harnesses, the animals should return to the stable on their own.