I live in Washington State and I bought a trailer, the guy that I bought it from gave me a title but I don't know if it's the right one and he never transferred it into his name

What makes you think its not correct? Have you tried to transfer the title to your name?

No because the guy that I got it from did not put it in his name

You may have been duped out of every penny for something you now don't own, so you need a lawyer ASAP.
Also go to city or town hall clerk of deeds and find out who owns it! You may even find someone who knows the real owner, if it's a small enough town.

So he had a title that was not in his name, but was it signed by the person who's name it was in? If it was, then you still should be able to register it. If it was not signed, then you may have been duped.