The Pack Rats are a trio of burglars who operate in the Hundred Acre Wood. The three pack rats have the bad habit of taking whatever they want from people when they aren't looking, although they do pay for what they take in some fashion, usually by leaving walnuts in place of the stuff they've robbed.
The Pack Rats are first seen in the episode, "Nothing But the Tooth", in which they steal a tooth which Pooh thinks it's his sweet tooth, and Pooh tries to take it back as he believes he won't be able to eat honey anymore without it.
The rats would later play a major role in the episode "The Rats Who Came to Dinner", in which Pooh agrees to let them stay in his house during a flood, as the rats' shelter had been destroyed by the water. In this episode, Pooh learns that the rats do not think of themselves as thieves (and are in fact shocked by this idea) as they always trade walnuts for what they take. Pooh, on his turn, explains to the Pack Rats the concept of friendship and how friends should respect the others' property and not take things without asking. At the end of the episode, the Pack Rats actually manage to save all Pooh and his friends' possessions from the flood, and a party is held in their homage.
The rats make their next and latest appearance in the episode "Oh, Bottle" where they try to steal a bottle in hopes of finding a treasure shown on a map, which is actually a message in order for Pooh and friends to rescue Christopher Robin. They succeed in stealing the bottle, but it accidentally breaks. The rats can't understand what the map says and gives it back to Pooh, who sadly states that without the bottle they can never use it to rescue Christopher Robin. Feeling sorry for Pooh and friends, the rats give them a new bottle from a collection of bottles they've been saving. They are last seeing bidding Pooh and the gang farewell as they head down river to find Christopher Robin.
In real life, the pack rat (Neotoma cinerea) is a rodent species native to North America which is indeed famous for its habit of collecting seemingly random stuff, including taking small items (spoons, watches, coins, jewels, etc...) from campers or travelers during the night and replacing them with another, usually more ordinary object such as a stick or a walnut. This so-called trading habit has been observed in other rodent species as well. Pack rats are also well known for building intricate nests with the things they collect, sometimes using small caves, burrows or hollow cactus (or even abandoned human constructions) to house their middens.