Pests should be more. . . pest-like.

[Call me Arty]

   While nearly all animals in Rimworld follow the same "wander until hungry" A.I. and could do with more similarities to a realistic counterpart (assuming they wouldn't naturally just wander until hungry), I think critters like squirrels, rabbits, and rats (probably megascarabs too) would benefit the rough survival feel of the game the most. When these humans are making nice, warm shelters with plenty of food and crops, who would want to sit around in Wolf n' Warg country?

   Squirrels: Would you believe me if I told you squirrels like trees? They're excellent climbers. This climbing wouldn't just apply to trees, however. Leaving a wall up with no ceilings on either side would allow them to simply climb over and into your compound, ready for your unprotected crops and food. Beware of trees inside your base, as squirrels would love nesting in fully-grown trees. Chop it down for some angry parents and fleeing pups, or keep it around for farming some little baby squirrels.

   Rabbits: Known, of course, for burrowing. If and when they want your sweet, sweet vegetables, they should tunnel up to two tiles from the dirt patch they're on. So if your only defense for your garden in a single wall and dirt/gravel on either side (dirt/wall or floor/dirt), the rabbits could tunnel through to the crops. To combat this, consider some tiling or second layer of walls on the outside of your protected area. These dens, could be filled in (like a grave, no materials, just kick some dirt in) which would force any occupying rabbits out the other end, or act as a source of young rabbits if you allow it to continue feeding.

   Rats: To quote the dev himself in it's description: "A rodent famous for soiling kitchens and spreading disease. Eats almost anything; lives almost anywhere." Yet, what do they do? Politely sit outside chewing on seeds and waiting for a bear to get hungry. Instead, they should tunnel into soft materials, such as wood (until we get something else, such as bamboo or canvas walls) or squeeze themselves under doors with no flooring on the same tile. Of course, your crops and meal fridge are at risk of this. When they tunnel through wood, they wouldn't destroy it, but instead lower its durability a bit and leave a hole for easy access later. It would also be interesting for rats to nest in some of these tunnels, acting as a source of baby rats, so a possible farm to balance a new invasive pest. Destroying it would release some very upset inhabitants while closing it would make the rat family flee out of any other exit.

   In addition to a new threat that doesn't need fangs or an assault rifle, they could act as a sort of tutorial, almost. Squirrels getting in? Build a roof over your walls, which can help in keeping snow from areas you might need to rush through or past. If you have some tiling and walls around your garden because lousy rabbits keep getting in, then it can help additionally teach you to set-up a wildfire/cold snap proof farm. The rats and their tunneling will promote reinforcing your walls with more than just a couple planks.

[Bakar]

+1
Great idea, it would give rimworld a more realistic feel and a new challenge (A challenge which doesnt threaten the existance of your colony)

[SpaceDorf]

Rats also climb nearly as well as squirrels and can chew their way through minor obstacles.
In addition to this Rats are really smart and the ones that survived on a Spaceship to get to a Rimworld should be even more so.
( With all the Terraforming I can't imagine that settlers actually introduced rats to other planets on purpose. )

The disease part of the Rat description is only partly true. Rats themselves try to be clean, but the conditions of large cities did not prevent such diseases and the Rats were immune and did not care.
Finally Boomrats should behave equally to their ancestors

All things said, I like the suggestion and love any challenge for a colony that does not involve bigger guns.
In addition to that it would empower cats and finally give yorkies a reason to live. ( at least ingame )

[Tyriq]

I like this idea. Rats could help spread diseases, making it feel more organic rather than a chance event (colonists get sick individually instead of all at once, and chance for sickness to occur depends on your cleanliness. It would also give more use to Sterile tiles (put some in the freezer to dissuade diseases). Rats spreading disease could also be devastating for tribal groups (as it was throughout humanity's history).

[warbrand2]

need this, the pests are just well annoying and once you get a wall built not a threat.... or tame a cobra.