It's sad to see people fighting about intellectual property rights when they apparently have no clue what those are, how they work and what is allowed (and not).
Prime example: boltstrueno1fanever.deviantart… vs. fav.me/d4vcima
Obviously the latter is similar to the first.
The question is: is this allowed?
To answer that question, it is important to understand what rules apply and what intellectual property (IP) is.
There are three different kinds of IP that can apply here:
As the name hints this is the right to make copies of a work. It does not protect ideas, only the actual recorded work - artwork, documents, music etc.
This means that if someone copies your art (by directly copying it, by tracing it or otherwise adapting your work) you can certainly take action, but there is little you can do to prevent someone else creating their own work based on a similar (or even the same) idea as long as they are not copying your actual work to do so.
If you are inspired by the work of others, that is fine and nobody can stop you, but when you create a work of your own, start with a blank sheet and do not try to copy what others have made.
In the Ox vs. Onyx case above, the character Ox is obviously based on the same idea as the character Onyx. However, it is also quite obviously not a copy of Onyx. This means there is no copyright infringement here.
Copyright does not apply to items such as names and titles.
Even if "Ox" is similar to "Onyx", this is not a case of copyright infringement. Not even if the characters had identical names would this be copyright infringement.
Names and titles and stuff like that is covered by a different area of intellectual property protection:
2. Trade mark
A trade mark (or trademark) is some kind of sign or indication used by someone marketing a product (or service) to distinguish this product from those of others. It is a way to protect a producer's or a product's reputation from being tainted by inferior copies.
A famous example: Coca-Cola
If someone else sold really cheap and bad soda labeled "Coca-Cola" (to make immense amounts of money), you as a consumer would think "eww Coca-Cola is bad" and not buy it. This hurts the reputation of the real product! It would simply not be fair to the real producer.
Trade mark protection makes sure that you as a consumer can know that the product you are buying is the real thing.
Hello Kitty is a good example: nobody can produce Hello Kitty pens, pen cases, backpacks, mugs etc. except Sanrio corp. (or those with their permission).
Trade mark infringement is why cheap copies of Gucci bags, Rolex watches etc. are illegal. If a consumer buys a cheap "Gucci" bag and it breaks after a couple of weeks (because it is of lower quality, or it wouldn't be so cheap!), that hurts the reputation of the real Gucci, who spend enormous amounts of money making quality bags. Not fair!
It is theoretically possible to register the character Onyx above as a trade mark for a class of products, e.g. candy, shoes, electronics, cars etc.
If that were done, nobody else could use that character or its likeness to sell products in the same class, if there was a possibility of the consumers confusing them with the "real thing".
Nobody is trying to sell e.g. Ox shoes to unfairly compete with all those Onyx shoes on the market. No trade mark infringement here.
A product's likeness can further be protected by a little-known kind of intellectual property:
3. Design protection
This is a protection of the appearance of a product, or simply the way a product looks.
This includes things as shape, colours, texture, materials and so on.
To claim design protection, the design has to be new. This means no identical (or very similar) design has already been published before.
It also has to have individual character. This means it has to be different from the appearance of all other already known designs.
Only a three-dimensional shape is protected unless you register your design. Colour patterns etc. are not protected otherwise.
In the Onyx vs. Ox case above, Onyx has a design consisting of the colour pattern, markings etc. This could theoretically be registered as a protected design for something, like a plushie for example, and then nobody else could sell plushies with that design - but 2D fan art would still be legal!
However, as it is not a registered design, there is no design protection for Onyx.
4. Original character protection
Oopsie, I said three kinds, not four, right?
Right. There is no such thing as "original character protection". Got you there, didn't I?
I see lots of times people making their own fan art, and then putting "character belongs to X", "OC Naruto © Kishimoto" or something like that.
This is nonsense. It is your work (unless you copied it!), not Kishimoto's. A name does not have copyright, and an idea does not have copyright.
And no, if you do actually copy someone's work (getting inspired and doing fan art does not count, as I just told you), adding "blah © original artist" at the bottom does not help, it's still illegal if you don't have permission!
On the other hand, if it's not a copy, then that thing just shows everyone how ignorant you are about how copyright works
Hopefully you feel more informed now!
In any case, always remember:
Imitation is the sincerest of flattery.
Charles Caleb Colton (1780-1832)