Rat

Rats are long-tailed rodents.

Sewer rat
A sewer rat is just another weak monster. Compare it to the kobold, the newt, or the grid bug, but those three monsters are far more popular.

To see why, try these searches on Dudley's dungeon, the webcomic set in NetHack:
 * newt returns at least 31 comics (thanks to the rivarly between Dudley and newts)
 * grid bug returns 4 comics
 * kobold returns 3 comics
 * sewer rat returns exactly one comic, 7 December 2005

So (at least for the authors of Dudley's dungeon), even kobolds have three times the probability of sewer rats. (If you look in Wikihack though, the sewer rat page appeared at 26 March 2006, before the pages for any of the three other monsters.)

Giant rat
A giant rat is one of the fabled "rodents of unusual size". It is nothing special.

Rabid rat
A rabid rat has an animal body. It has no hands. It is poisonous if eaten. It is a carnivore.

SLASH'EM
In Slash'EM, if you throw cheese at a sewer rat you might tame it, but it's probably not worth the effort.