Truth is, in the US, the artist retains all rights unless they are specifically given away. You can't even "accidentally" give rights away. There are only 10 reasons why an artist would lose the rights to things they create (Not counting a written agreement to give rights away). If you aren't an employee, then there are only 9. If it's not a motion picture, only 8. If it's not a test or related to a test, only 5....

See here:
 
The items on the left are the 9 reasons an independent contractor would not retain rights to a creation by default

William Fisher has provided a quite lengthy series of lectures on YouTube regarding US copyright law.

While the question is design related, any direct answers would need to be legally based, so..... you need to speak with an attorney.

Is it copyright infringement by US copyright law if someone else modifies and uses my design?
If you summon a creature and give it any item then dismiss it, does the item go with it?
