Rats are another humanoid faction that are similar to Bandits, although they have their own variants that differentiate from them. Some Rats will stop fighting in order to pick up loot from the battlefield, even going as far as to bury it, making it so you can never get it back. They can also feed on corpses to recover some health. Like with Bandits, they are weak in the head.
The Tunnel Rat is the most basic of the Rats and definitely the most common. They tend to stay away from you and shoot at you with their Assault Rifle, while also dodging your own attacks. Be careful whenever you close to them, as they can deliver a melee strike. They can also pick up and bury loot.
The Field Rat is a tougher version of the Tunnel Rat, with the same type of attacks in their arsenal. However, they can sometimes do a leaping attack (similar to the Bullymongs) to get within melee range, then pull off a quick strike. They can also pick up and bury loot.
The Rat Thief doesn't seem to concentrate on fighting, instead opting to focus solely on pickpocketing money from your own pockets. If they do manage to get some, they will scamper off to bury it and should that happen, it will be gone forever.
This is a smaller version of the Tunnel Rat that can be harder to hit, due to its small size.
The oddball out of the group, the Lab Rat is a very slow-moving foe that doesn't worry about anything but putting the hurt on you. They have extremely high health and once in melee range, they will deliver a painful strike. If they can't happen to reach you, they will toss a hatchet at you, similar to Psychos. It is best to bait them into chasing you while keeping your distance, making sure that you don't stay in an area they can't normally reach.
This is another variation of the Tunnel Rat, but is only found in specific locations (Tundra Express and certain sections of Lynchwood). The only difference is that it can dig underground, where it can't be harmed, popping up to attack you before retreating underground again.
Exclusive to TVHM, this Rat will move quickly on all fours to swipe at you with corrosive-soaked claws.