Apart from the bugs and reptiles that populate the regions of Rain World, there are also multiple mammals that perform vital functions despite not being high in the food chain.
These beings are very different from each other, from colossal creatures to beings smaller than the Slugcats themselves.
Rain Deer are tall, passive creatures found exclusively within 
Farm Arrays. They serve as the primary method of crossing the large fields of 
Worm Grass found throughout the region.
Rain Deer have large bodies with an oval head, two yellow eyes, a set of antlers, and four long legs. Their legs are thinner and generally lighter in shade at the tips and have four distinct joints. The underbelly of the Rain Deer's body has several hanging appendages of varying lengths. Similar appendages may also be seen on the antlers, although much thinner and shorter.
The player can grab and ride on a Rain Deer's antlers by pressing Up. The 3 simplest ways of getting a hold of them are:
Spore Puff, Rain Deer often sit down near the spot it was thrown, providing an opportunity to grab their antlers. Though unreliable, waiting for a Rain Deer to sit down by themself also works.Rain Deer slowly wander across rooms and ignore other creatures. Sometimes Rain 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 beneath their body. They behave a bit differently when ridden, often stopping at potential room exits, giving Slugcat time to get off their antlers. While they can be killed by Worm Grass if stunned, they usually escape easily if grabbed by it. When the rain comes, Rain Deer leave by moving out of bounds.
Rain Deer are attracted to Spore Puffs. When one is thrown, nearby Rain Deer emit a loud shriek before moving toward the spot where it was thrown. If they find one on the ground, they bow down in an attempt to eat it.
Rain Deer are affected by the dominance trait; ones with less dominance bow down to those with more dominance, letting them pass.
Lantern Mice are passive, fearful creatures found in 
Shaded Citadel. Their natural bioluminescence may be exploited to navigate dark areas.
Lantern Mice are rodent-like creatures with large eyes. They are around half of the size of 
Slugcat. Their backs sport four big dots, two on each side. There are two main varieties of them by color: pinkish-orange and blue. The varities are equally to encounter. A Lantern Mouse has a 1% chance to spawn with a completely random hue instead of picking between these two varieties, however this doesn't exclude the selected hue from falling under two varieties. The two common varieties take up 25% of the possible hue values in total, resulting in a 0.75% chance for a Lantern Mice to have an appearance distinct from either of the two common varieties.
Their default state is glowing, with their colors being bright and vibrant. Occasionally, a Lantern Mouse attaches themself to the ceiling and goes through the process of lighting up. The Lantern Mouse's main body fades to complete black while the spots on their back turn into a saturated version of their main colors. Their eyes also invert to white while this takes place. After a few seconds, the Lantern Mouse abruptly reverts to their normal color scheme with a small burst of particles expanding outwards.
Lantern Mice are fearful of most other creatures and flee when approached. When idle, they either wander around or sleep by hanging on the ceiling by their tails. Lantern Mice tend to be seen in groups.
If injured, Lantern Mice struggle to glow at full strength. If killed, they continue to glow for a substantial amount of time and may be used as a temporary light source. Eventually, a dead Lantern Mouse completely fades.
Slugcat is able to grab on to a Lantern Mouse as they hang from the ceiling; however, they do not immediately detach in most cases. There are ways to force a Lantern Mouse to detach their tail, the easiest being to enter a pipe into another room, which always severs connections of any kind.
While their glow repels small groups of Coalescipedes, due to their low weight, if a group of 
Coalescipedes manages to grab onto a Lantern Mouse it almost always results in death.
Yeeks are skittish, colorful creatures with two long tails that jump around open spaces and are commonly found in groups.
Yeeks are round, squat, roughly Slugcat-sized creatures with two stretchy tails, a pair of powerful legs, and two large, round eyes. Each of their tails have a single brightly colored stripe partway along their length and end in a round 'club' of the same color. Their two tails have different lengths, although this is not always evident due to their elasticity and constant motion. They may have spines on their backs, which can be sharp or clubbed.
Yeeks can be carried to allow Slugcat to jump high into the air, or to climb vertical poles much faster by jumping. Backflips also travel much higher while holding a Yeek. While carried, Yeeks stop jumping normally and let the player take control, though they still perform small hops independently. Yeeks require two hands to hold, making self-defense difficult while using them.
Yeeks jump as long as JUMP is held (not pressed), Down is not held, and the player is either in contact with a floor or on a pole. Additionally, there is a short cooldown on when a Yeek can jump, which is even activated by their small non-player-initiated hops - this means that pressing jump while standing on the ground frequently does not result in a boosted jump. Actions such as pouncing are often impossible to perform while holding a Yeek, though slides are possible.
Yeeks are relatively unmotivated when undisturbed, typically gathering in a single location and hopping up and down while looking at each other. Though they do not actively search for food, they can pick up most food items, but they do not eat them or take them to their dens. When disturbed by hostile creatures, they begin jumping high into the air and attempt to flee from the threat.
Yeeks are capable of making short hops to move horizontally, jumping very high into the air, slowly climbing poles, and slowly crawling through tunnels. However, they can have difficulty traversing the environment due to their overall lack of coordination, usually performing seemingly random actions without any clear direction, or getting stuck.
Yeeks can theoretically use their high mobility to avoid predators, but their strongest defense is usually jumping up and down in a single spot, which is only mildly effective against Worm Grass and ineffective against most other predators.
The base of the Monster Kelp remains stationary, but their flexible stem can move and attack other creatures. Monster Kelp remains still while no creatures are in sight. Upon detecting prey, Monster Kelp stretches towards them and follows their movement with the tip of their stem. If the tip makes contact with their prey, the creature is dragged into the Monster Kelp's den and killed. Monster Kelp hunt almost all creatures.
Monster Kelp have movement-based vision which takes the form of a full radial view from the tip of their stem. They are completely unable to hear.
RAIN WORLD WIKI