Rain Deer have large bodies with an oval head, two yellow eyes, a set of antlers and four long legs. Their legs are thinner & generally lighter in shade at the tip, and are made up of four distinct joints.
The Slugcat can ride Rain Deer by grabbing onto their antlers with the same controls as grabbing a pole (by pressing up). The three simplest ways of getting a hold of them are:
Getting onto a Deer's back while it's sitting down. When attracted by a thrown Spore Puff, Rain Deer often sit down near the spot it was thrown, providing the perfect opportunity to grab their antlers. Though unreliable, waiting for the Deer to sit down by itself also works.
Dropping a Spore Puff for the Deer to eat. While eating Spore Puffs off of the ground, Rain Deer bow their heads in a way that makes their antlers easily accessible from the front. There is a short window that the Slugcat can regrab the Spore Puff while also boarding the antlers; this does, however, require quick reflexes.
Crouching down while in view of the Deer. In response, they bow, giving the opportunity to jump onto their antlers.
Due to Rain Deer frequently staying in one place or moving in the wrong direction, the player can wiggle their controls back and forth in order to prompt the creature to begin moving. Additionally, wiggling causes it to change directions if already in motion.
Moving across Worm Grass with the Slugcat on its back
Rain Deer ignore other creatures, most of the time slowly wandering across rooms instead. Sometimes Deer may stop moving in front of an obstacle, before continuing to walk with a small burst of speed. Upon reaching the end of a room, they may choose to sit down, hiding their legs in the process. They also behave a bit differently when ridden, often stopping at potential room exits, giving the Slugcat time to get off their antlers. While they can be killed by Worm Grass if stunned, for example by a Spitter Spider's spit, most of the time they escape easily if grabbed by it. When the rain comes, Rain Deer leave by moving out of bounds.
Rain Deer are attracted towards Spore Puffs. When one is thrown, nearby Deer emit a loud shriek, before moving towards the spot where it was thrown. Alternatively if they find one on the ground, they'll bow down in an attempt to eat it.
Rain Deer's antlers vary in shape and length, the latter of which is determined by the dominance trait. The appendages that hang from their bodies and antlers vary in amount, length, position and thickness.
If a Rain Deer is about to move out of bounds while ridden, they begin shaking wildly in attempt to throw the Slugcat off.
Rain Deer have a reputation system that can be viewed with dev tools, but is ultimately unused, as their behavior is identical regardless of how high or low it is.
While there aren't any in-game ways to increase it, reputation can be decreased by killing Rain Deer.
Damaging Rain Deer with Spears pushes them forward.
This allows for a strategy sometimes used in speedruns as the Hunter, where two Spears are repeatedly thrown at a Rain Deer while it's being ridden, providing a massive speed boost.
It is possible to ride a Rain Deer as Saint while grappled onto the body of another Rain Deer. This causes the grappled Rain Deer from moving away from the Rain Deer that Saint is riding, and pulls it along when Saint's Rain Deer moves.