This happens WAY more often than it should, and I really want to discuss this.
With the creativity and freedom for anyone to create a closed species, it is only natural that people are going to have a few similarities between not only a CS, but one-off characters too. Some people are going to have more original traits than others, but regardless, you should never harass or threaten anyone who has a character that have traits of your species. You do not and never will own the concept of a trait. You only own the species and lore of characters that have the trait. It is a totally different story if someone flat out copy every aspect of your character to a tee, but otherwise, leave the person alone! When you sell MYOs, you are selling the right to create the character in the context of your species's lore, along with entry to your species's Community. You are not selling rights to a trait. You do not own doll joint kemonomimis, jelly horns, or water limbs. You do not own angel wings or randomly placed eyes either.
Even if a person has a character with your exact species traits, as long as they are not ripping off a specific character, please leave them alone. Chances are, they don't even know you exist. Either that, or they like a trait or two, but do not feel invested enough to join your community. It is completely natural to take inspiration from other artists and do your own thing with it. There is even a chance you took inspiration from another person to make your own species or characters. They are not claiming to own one of your species, nor are they replicating your lore, so why does it matter?
To conclude, if someone has a trait on a character that you make a species that happened to have that exact trait, please leave them alone. They are not hurting anyone, nor are they claiming that the character is part of your species. You do not own traits, and asking someone to change their character just because they have a fish tail on their head or something is very self centered. If you do this, please stop. They are not hurting anyone. They are just an artist that want to have fun, like anyone else.
If you want to discuss this further, please leave a comment below! I am always open to new view points!