Re: When Ash gave Sabrina his Haunter, why didn't it evolve?
I don't think giving Sabrina Haunter really counts as trading. There are trade machines in the anime so maybe they have to be used. Ash used one to trade (and trade back) his Butterfree for a Raticate. I know he traded with Dawn during Sinnoh but I haven't seen that episode so I'm not sure if they used a trade machine or not...
I haven't seen Sinnoh episodes, but as far as I know, they haven't shown a Pokemon that evolves by trade just evolving normally, have they?