Wererats are people who transform into creatures that are half-rat, half-human. They're firstly mentioned by Geralt during is conversation with Dorregaray in the short story The Bounds of Reason, where, according to him, an old hierophant in Rivia had been killed by these beasts.
Wiedźmin: Gra Wyobraźni
They are only type of lycanthrope to attack in the herds, in groups of two to four of them.
They feel good in the company of rats though they do not like much pseudorats. They have an aversion towards werewolves and totally hates werecats and normal cats.
The Witcher (PC)
They were supposed to appear in game as monster lurking in sewers of Temerian capital Vizima but were later replaced by drowners.