MISP project maintains an exhaustive list of threat actors with metadata, relationships and synonyms. The format is machine-parsable and all is open source under CC-0/2-clause BSD license. Feel free to reuse and/or contribute.

https://github.com/MISP/misp-galaxy/blob/master/clusters/threat-actor.json … -
https://www.misp-project.org/galaxy.html#_threat_actor …pic.twitter.com/lEVgeLgU2o