Was made by shocktopus1717 (as credited in the screenshot), who is apparently
f0remsics on Reddit. Created 105 sprites for Infinite Fusion as of writing this.
Introduced officially fused Pokemon long ago, technically. See: Necrozma + Lunala/Solgaleo and Black/White Kyurem. Argues Iron Valiant to be a fused Gallade + Gardevoir. But, as a general thing for all Pokemon? No way. Imagine a modest PokeDex of 50 Pokemon. Will say you cannot fuse a species with itself and you can only fuse them one way (unlike with Infinite Fusion). Leads to 50 * 49 = 1225 different combinations. Given the reduced PokeDex size in the newer games (400 base game), not to mention 3D models, do you think the Pokemon Company will do that?
Probably can hack it into the games...somehow. Adds official Pokemon to ROMhacks pretty commonly. How? Zero clue. Likely involves a good bit of know-how.
Downloaded and found the sprite in InfiniteFusion's folders. Cropped it out of the sprite sheet. (For reference: CustomBattlers > spritesheets > spritesheets_custom > 439. Then 439.png.)
Edit: Bonus: Added the whole line below, plus an alternative Gengar + Trevenant fusion. Credits: holomew151 for the Gastly fusion, johlly for the Gengar fusion, and lekibun for the Gigantamax-inspired Gengar fusion.
Edit: Fixed the number of combinations. choose(50, 2) = 50! / (2! * (50 - 2)!) = 1225. Or, alternatively, (50 * 49) / (2 * 1) = 1225.