unfortunately, you cannot change the name of a pokemon you traded from someone else. If it's really that bad, you could trade it back, ask them to trade it, and get it again.
You could also breed to get another Pokemon of that kind (in this case, Snivy, then evolve it) to get Serperior, but of course, that takes time and I don't know if the Serperior you got was EV trained, had a specific moveset or ability, or whatever.