Monster origins: Celtic monsters

[Huw]

I've been looking into the folklore origins of several creatures, and I've drawn a blank at several well known Celtic monsters. Does anyone know of pre-19th references to the following:

Fachan/Athach: Their physical description resembles certain Fomorians, but I cannot find either more recent name.

Nuckalevee.

Water Leaper/Llamhigyn y dwr.

Thanks in advance!

 

[HalWhitewyrm]

For the Fachan, I wrote a product for my Bardic Lore series specifically for that creature.
Bardic Lore: Fachan

The write-up is based on a variety of actual sources, including some pre-19th Century ones. In fact, that whole series is based precisely on presenting Celtic elements for gaming, but drawn on the actual sources as opposed to popular fantasy tropes. Take a look at the other releases as well (and today, Sun. June 27, they are still 25% off!).