Since you specifically state "Black 2" this does not include the Pokemon that are exclusive to White 2 (Electabuzz is the only one that fits this). Also it does not include Pokemon found in the post-game. This is because in the post-game you have the National Dex, and so can theoretically get any Pokemon at all.
- Magmar: Evolves into Magmortar when traded holding the Magmarizer.
- Boldore: Evolves into Gigalith when traded.
- Onix: Evolves into Steelix when traded holding the Metal Coat.
- Gurdurr: Evolves into Conkeldurr when traded.
- Karrablast: Evolves into Escavalier when traded with Shelmet.
- Shelmet: Evolves into Accelgor when traded with Karrablast.
As far as I can tell, that is it.