Honestly, when I compare The Mysterious Mare Do Well to Mr. Dark, I see a few similarities.
1. Colour Scheme (Basically the same!)
2. Hat Design (Quite similar, if you compare them.)
3. They both have capes of similar design/colour (Though I'll admit that Mare Do Well's cape is noticably longer.)
I know, it's only 3 similarities I've listed, but admittedly they are quite significant. I personally think Mare Do Well looks like a pony version of Mr. Dark.
Here, see for yourselves!