Tylwyth Teg 

The name for the fairy folk in the Welsh folk tradition is Tylwyth Teg.
Though usually small in size, there were also said to be rare instances in
which they had the size of ordinary humans. This special sort of Tylwyth
Teg was said to come out of lakes and dance, and their fine looks enticed
young men to follow them back into the lakes, and there marry one of them.
If the husband wished to leave the lake he had to go without his fairy
wife. They were often seen riding out of the lakes and back again on
horses.

Some lived in caves; others of them lived in lake-bottoms. A farmer one
morning found in his field many very fine cows such as he had never seen
before. Not knowing where they came from, he kept them a long time, when,
as it happened, he committed some dishonest act and, as a result, women of
the Tylwyth Teg made their appearance and, calling the cows by name, led
the whole herd into the lake, and with them disappeared beneath its waters.

- O’Gravy