There are a number of different tales of the Afanc which either describe it as a water demon or a creature that looks like a mix between crocodile and beaver. In all cases, however, it was a dangerous creature which would prey on those who went into its lake. In one of the more interesting stories about the Afanc it kills three of the kings sons (chieftains) every day when they go out to slay the it and everyday the court maidens bring these sons back to life. It's important to understand at this point that certain woman had the power to control fate and so can bring people back from the dead.
After the kings sons had died and come back to life for a long time a man named
Peredur asks to go out with the three chieftains, but they refused as
they wouldn't be able to bring him back to life. Determined Peredur
strikes out on his own so that he might slay the Afanc and thus
increase his own fame and honor. It's important to keep in mind here that heroes in ancient Europe were not necessarily what we would think of heroes should be today, much of their motivation was purely for glory and fame.
On his way he meets a maiden (The
Queen of Constantinople who is most likely a stand in for what was
previously another fairy figure or shaman figure). This 'fairy or
shaman' gives Peredur a stone that allows him to see things which are
invisible for the Afanc as it turns out has this ability, just as it
has the ability to shoot poisoned darts at it's victims.
Still Another Tale the Afanc acts like a unicorn and lays it's head in
a maidens lap allowing the villagers to capture it. For me what's interesting about the Afanc isn't so much the monster but that he is an anthropomorphic beast which seems to be a remnant of shamanism in Europe.