Situation: An American male has been offered a job in Thailand, and the company that hired him is claiming that they can get his American female girlfriend a "long term partner visa".

Question: Is there such a thing as a "long term partner visa" or do the two need to be legally married so the female can be attached to the male's visa?

married

They must be married for her to qualify for a dependent extension of stay based upon his extension.

You need to be married. In some cases a long-term partner is able to get a multiple entry non-O visa from a friendly consulate, but such cases are rare and immigration will not give any extensions.

Thank you all for the responses.