Not 100% sure but I think an "agent" is a simpler kind of AI unit.
Meaning they can move and even react to certain stuff, but not much more.
The german sheperd (alsatian) is an agent for instance, and it barks and runs when you shoot at it.
It can also be spawned as a unit, by createUnit command (thanks DeanosBeano), but it still does not work perfectly as a group member. It often gets stuck when moving etc.
Seems like the "agent" models are not perfectly created as a normal AI unit.
Laggy