Question

[dark_croconaw]

Can you evolve Giovanni's Nidoran into a Nidorino? Or can it only evolve into Giovanni's Nidorino?

 

[Pokedragonfire]

Only Giovanni's Nidorino, not plain Nidorino.

 

[dark_croconaw]

Also can a dark pokemon evolve into a non-dark.
Example: Dark Haunter evolves into Gengar?

 

[Kairyu]

Nope, Dark pokemon can only evolve into other Dark Pokemon. Same for the wild and gym pokemon.
Examples:

Misty's Horsea > Misty's Seadra

Team Magma's Aron > Team Magma's Lairon

Dark Wartortle > Dark Blastoise