An Implication from the SRD 5.1 being now under CC-BY 4.0 [NOT LEGAL ADVICE]
Thanks to WotC putting the entire SRD 5.1 under Creative Commons… They did do a slight blunder. They have surrendered certain terms that were previously deemed product identity!
beholder (listed as a typical aberration and as an example card in the Deck of Illusions)
mind flayer (also listed as a typical aberration, as well as when describing the Psychic damage type)
umber hulk (listed under the Guards and Wards spell and the Tremorsense entry)
All three were explicitly mentioned in the old SRD 5.1 that included the OGL 1.0a preamble as being part of product identity.
Arguably also included in this list are the following two entries, since they are "the vampire Count Strahd von Zarovich", who is mentioned in the SRD as an example for the paladin's divine sense feature, and the slaadi, who are also included in the list of aberrations together with mind flayers and beholders.
Now what does this mean?
For one the stat blocks for any of these creatures is not available under CC-BY 4.0, and Count Strahd von Zarovich is a proper name of a developed fictional character. So, as a not-lawyer, I'd recommend you avoid using Strahd for any work derived under the CC version of the SRD 5.1 because that is not going to end well.
Now in regards to the slaadi, beholders, mind flayers, and umber hulks... You can definitely now mention these terms in your published homebrew (as long as it uses proper attribution under CC), so if you ever wanted to write a mind flayer adventure, you can now sa-fely create an encounter that includes a fight between the heroes and a group of four mind flayers led by an arcanist (with the clear implication that the DM needs to check the Monster Manual for the stat blocks) or similar. Creating custom stat blocks for these creatures, especially for commercial projects, is not advised.
Furthermore you can PROBABLY use these terms to label similar creatures in your original non-D&D works, as long as it also includes the proper attribution. Also I'd still suggest making an effort to make them distinct enough, so e.g. with beholders, yes to alien eye monsters, probably no to the typical arrogance and eye stalks.












