Son Had Kardashian Last Name at Birth

Khloe Kardashian wasn't messing around when her son was born, giving him the Kardashian last name instead of Tristan Thompson's ... but later had a change of heart.

Sources with direct knowledge tell us Tatum Thompson -- as he's now known -- was initially legally a Kardashian. Our sources say Tatum also wasn't his first name, though we're not sure what first name Khloe originally chose for the kiddo.

Eventually, our sources say Khloe decided to drop the Kardashian name for Thompson, and that's now Tatum's legal name moving forward.

As for why Khloe may have made the initial decision to leave Tristan out of things -- it seems pretty clear ... as we reported, Tristan got another woman, Maralee…