I looked around to see the copyright status of the name "Derro". Wizards of the Coast does not claim it under their "Product Identity" trademark list for characters, which is as follows: "Balhannoth, Beholder, Carrion Crawler, Displacer Beast, Gauth, Githyanki, Githzerai, Kuo-Toa, Mind Flayer, Illithid, Slaad, Umber Hulk and Yuan-Ti" (
Source). The Derro is also present in the SRD, and Pathfinder makes use of the creature. Now, this could still mean that only companies abiding by the OGL or the GSL could make use of the Derro.
But other companies have used the name "Derro" unchallenged, without (to all appearances) having a license from WotC. For example, these miniatures from Reaper Miniatures:
https://www.reapermini.com/Miniatures/D ... test/77330https://www.reapermini.com/Miniatures/D ... test/77332https://www.reapermini.com/Miniatures/D ... test/02945And these Derro miniatures aren't for their Pathfinder miniature line, either, but for one completely independent of that. Furthermore, the Derros are based on the "Deros" of Richard S. Shaver's "I Remember Lemuria", further weakening any claim to copyright on the creature WotC could have. Shaver's estate could hardly have a claim upon the Derros, either, having let WotC's use go unchallenged for so long, and since "I Remember Lemuria" is in the public domain (because its copyright wasn't renewed).
So it seems that the name "Derro" is safe to use.