What source does this appear in ?
Ferocious Fetch (Ex):At 5th level, a mad dog's war beast gains Improved Drag as a bonus feat. As a swift action, a mad dog can command her war beast to move toward an opponent within 30 feet and attempt a drag combat maneuver to pull the target back toward the mad dog. The war beast can move before and after performing the maneuver, but this movement counts toward the war beast's total movement during its turn.
This ability replaces improved uncanny dodge.
Using the LINK(Ex) ability of the AC, handling is usually a free action and pushing is a move action.
Handling an animal prompts it to use one of the tricks it knows. There *IS* an existing "Fetch" trick, but Ferocious Fetch isn't fetching an object, it is attempting to pull a creature.
Think of Ferocious Fetch as a sort of Improved Trick. Instead of a move action to push, it's only a swift action, but not a free action like a regular trick.
EDIT: Careful, Sundark. "Immediate Action" is a rules term, and I don't think you're saying that the AC has to expend an immediate action. I think you're saying that the AC acts out of its turn as soon as the command is given at no action cost. I'm not certain that's intended, but that is one way of reading it.
If I were GMing, I'd say the AC executes this special command with its regular actions as soon as possible.