This is an artwork of two Raichus. One is Mega Raichu X and Mega Raichu Y.
Both these forms were revealed in the DLC trailer. https://www.youtube.com/watch?v=ExUYVdlA6eI
The origin of the pose was from this.
I will post more so stay tuned.
Mega Raichu X and Mega Raichu Y © Pokémon
Pokémon © Game Freak/Nintendo/Satoshi Tajiri