Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
IAN
As described in historical dictionaries
Patronymica Britannica (1860)
TREG IAN. Lands so called in the parish of St. Eue, in Cornwall, are supposed to have given name to this family. C. S, Gilbert's Cornwall.