I used to think that a trade - evolution Pokémon will evolve by a level up when it is away from the OT. Thus I thought that the Onix which we get for a Bellsprout in G/S/C(?) is awesome. Turns out I was wrong since I increased five levels after equipping a Metal Coat and no Onix lol.
But probably the Makuhita for a Slakoth was taxing. I mean, we all know how hard it is to find a Slakoth, right? And the nerve!
Hmm personally the Haunter was a real well-thought-of trick but I didn't fall for it (lucky me) because I knew something was fishy lol.