[quote=DimCarcosa]
Deoxys, as far as I know, didn't exactly need science to form, as it's just an alien space virus. Feel free to correct me.
[/quote]

Omega Ruby Pokedex entry: [i]The DNA of a space virus underwent a sudden mutation upon exposure to a laser beam and resulted in Deoxys. The crystalline organ on this Pokémon's chest appears to be its brain.[/i]

The ones from the original R/S/E Also say this, though they're the only ones. The rest just say that it was a virus that underwent a mutation when it fell to earth. So I guess it's just up to whatever you want~

He just seems a bit too....futuristic to be in a medieval setting to me, honestly. But, it's your call~