Water Horses and Other Fairy Steeds.. By: Aisling Bronach
Published in Traditions Magazine, Volume 1 – Winter 2004

Another reference on the subject:
Water faeries


And a final reference on the subject:
Merrows and Selkies

The top blue Clicky leads you to a very good discussion on the matter. The other two are okay. The Aisling Bronach article is very complete and a good starting point.