Brawn Rats in the TV series
The Brown Rats are vicious rats who move from the sewers to White Deer Park. They invade the park with the intention of taking over the whole reserve for themselves. They are led by a large white rat named Bully.
Books
The rats gradually arrive in the park and are slowly noticed by the park's inhabitants and the humans who are catching animals to move to the new reserve. The rats decide to retreat from the park temporarily so the humans believe the threat has gone, and they eventually return to take over the reserve. The Farthing Wood animals launch several attacks on the rats to drive them out of the Hollow, but the rats keep multiplying and are able to invade much of the rest of the park.
The rats invade the pond and cause havoc, killing several of the edible frogs and Bully also kills Toad, who tries his best to stand up to them and force them to leave. Many rats are killed by Tawny Owl and also Adder and Sinuous, who attack the rats in their nests. Bully sets up a trap and Adder is caught, but Plucky arrives and negotiates his release by threatening to dig his way into the nest. After this Bully offers Plucky a truce so that the two sides can raise their young and regroup, and plans to attack the Farthing Wood animals later on. The rats search for Adder and kill Sinuous instead, although the snake is able to strike and kill Brat before she is mauled.
The Warden is finally alerted to the rats' presence and lays poison for them, but the rats are too experienced to eat it and it achieves nothing. All the rats later gather together, forming a huge army, and launch an attack on the Farthing Wood animals in their corner of the reserve. The rats have the upper hand until animals from all over the reserve arrive to join the battle. Spike kills Mossy at the start of the battle, but is then killed by Adder, and Bully is killed by Vixen and his body is tossed over the boundary fence. With their leader dead, the remaining rats finally retreat and leave the park to return once again to the sewers.
TV Series
In the TV series the rats are almost the same as in the books. However, in the TV series Spike is not nasty like the other rats and eventually becomes friends with Toad (who does not die in the TV series) and joins the Farthing Wood animals. Also in the TV series Bully is not killed by Vixen; instead his tail is bitten off by Cleo, Weasel's daughter. This causes everyone around him including all the rats to laugh at him, and he, Brat (who does not die the TV series) and the other rats go back to the sewers.