A lake monster from Welsh Mythology. They would prey at anyone who swims or fall into its lake. They are described to look like a crocodile or a beaver so we could assume its big, hairy with sharp teeth.
They are said to be able to cause flooding whenever it thrashes violently a bit like how the Namazu, which is a mythical giant catfish in Japanese folklore which can cause earthquake whenever it becomes restless.
There is one story of the Afanc about a maiden who managed to get the creature to sleep on her lap. So while it was sleeping, fellow villagers chained and bound the creature which probably woke it up and actually ended up killing the girl in the process when it was struggling to get free.