Brine rats are Slayer creatures that require a Slayer level of 47 to harm and can only be fought after the completion of Olaf's Quest. These completely bald rats are fairly weak, but can be oddly accurate for their level. They drop very little of interest, mostly fish, coins, and massive amounts of rat meat (sometimes even noted).
They're notable for the fact that they are the only known monster to drop the brine sabre which is interesting only in the fact that it is rare and that it's special attack can be used underwater. In general, a Slayer assignment of brine rats should be considered fairly straightforward.