Rat mounts are a type of ground mount.
The [Ratstallion] is a reward from [Underbelly Tycoon].
[Squeakers, the Trickster] can be purchased from the Blizzard Shop or the In-Game Store.
Squeakers, the Trickster
[Sarge's Tale] is a free reward for completing the Mercenaries's tutorial in Hearthstone.