Rats run away at 5 or less hp, can be pushed by players or stronger monsters, can't push other monsters or boxes.
Killing those small creatures is the best way for a newcoming unequipped player to earn some money and experience.
You can also sell dead rats to NPCs Seymour, Billy and Tom on Rookgaard for additional cash. Also Baxter in thais buy rats for 1gp. It is very curious that, even tough they are probably the first enemy you fight when you start playing, they are the enemy which, in average, kills more players daily.
Rookgaard and Mainland, in most sewers and caves near towns. They can be found almost everywhere in Tibia.
Any Vocation: Just attack them, they shouldn't be a problem to a level 8+ (if it isn't a raid). No Vocation: Be careful with them, if many rats come to you, try to stand in a place where only 1 or 2 rats can attack you and try to avoid getting trapped by them.