Glen/Glenn was accepted by CDSNA as a sept in December 1988 based on its being noted as a sept in the 1954 publication Badges of the Scottish Clans. This sept name has a strong connection with Agnew above. Also listed as a sept of clan McIntosh.
Relationship to Douglas undocumented.
The most likely reason for this surname’s inclusion as a Sept of Doulgas is that it could be (arguably) a shortened form of Glenndinning. Research time was devoted to determining the veracity of this assumption but nothing of substance was found. Perhaps future research will shed more light.