In-gameEditRats only appear in Act V, almost exclusively in the Winding Cave, Plague Tunnels and Repository of Bones. As of patch 2.3, one can also find them in Ruins of Sescheron in Act III.
They attack in groups (5-10), and Rat Callers often summon more at will. Their attacks deal additional Poison damage, and rats move very quickly.
In addition to gray rats, green glowing rats also exist, named Explosive Vermin. They do not attack directly, rather jumping high into the air and creating a suicidal blast of Poison damage on impact. The radius is small, and they cannot change the flight trajectory (that is, one can easily outrun the falling rodents), but they can easily jump over any obstacles, reaching a player who is surrounded by normal Plagued Rats.
Both variants may be massively spawned by Rat Kings.