The default destination for anyone riding a pet through a transit, which Carl overrides for Gonk.
Because Dong Quixote was riding Gonk rather than walking through the Saferoom door, he would have gone straight to the pet stables.
Carl overrides that and orders him to Shanty Town instead, which has an exit appropriate for a Swamp Yak.
The routing rule Carl works around to keep a mounted fighter in the fight.