MUGEN is a highly customizable 2D fighting game engine. Because it is open-source, there are hundreds of different "versions" of Anime Crossover games created by fans. "V2.6" likely refers to a specific build by a creator (often found on YouTube or gaming blogs). These games are typically free fan-projects and should not be confused with official commercial products.
To spice things up, the creator added non-anime fighters: Anime Crossover MUGEN V2.6 Tai xuong Android PC