A gatekeeper at King Arthur’s court, and a servant of the warrior Glewlwyd Strong Grip.
Apparently, he was not a very good servant: When Glewlwyd’s other servants were killed, Glewlwyd despaired because Llaesgymyn was the only one he had left, and Llaesgymyn was “of no use to anyone”.
Sources
Culhwch and Olwen | Late 11th century
Geraint and Enid | 13th century