Lavellan

Lavellan are malicious river-monsters found in Scottish folklore, they are both fanciful accounts of real animals (such as rats and voles) as well as symbols of pestilence, due to the fact rivers were often a major source of water for communities the presence of plague and similar afflictions caused great devastation across the world, thus monsters such as the Lavellan were often created to act as a source of blame for these catastrophic events.

History
Lavellan are large rodent-like monsters from Scottish folklore that existed near rivers and was infamous for its extremely noxious presence, capable of killing cattle over a hundred feet away - they were usually depicted as giant rats, water voles or shrews.