M.U.G.E.N (also known simply as MUGEN) is a freeware 2D fighting game engine designed by Elecbyte.
In MUGEN, you can "create" your own fighting game. You just need to get the stages and the characters. Then if you want, you can also customize your own MUGEN.
Most of the stages and characters are gotten from fighting games such as Marvel Vs Capcom, King of Fighters, Street Fighter, Guilty Gear, etc. (at least the sprites and sounds), then MUGEN editors rip the sprites (or even customize their own sprites), code the animations, add the sounds, code the fighter's behavoir among other things to make the character playable on MUGEN.
You can download MUGEN from the official website: http://www.elecbyte.com/
After having the game, you'll just need to fill it with characters, stages, and such.
Here's a link where you can learn how to add characters: http://www.wikihow.com/Add-Mugen-Characters
And well, yeah, there are some characters like 'Kuromaru', 'Slime' or 'Minotaur' which were made for ~other purposes~ more than just fighting.. ^^ They are called NSFW characters.
My goal here is to increase the number of NSFW male characters. ^^
If you're new, you may want to download the current version of MUGEN (MUGEN 1.1b) I use.
UPDATE ~ July 2017 ~
This download link includes every compatible released character done by me on this site up to July 2017. Stages are included but they are soundless.
Have fun :)
If you want to be part of my supporters, you can do it at my patreon.
My supporters vote to choose which character will be getting new NSFW animations, plus having them first.