I came across a giant a little northeast of Mistwatch, staring at a dead mammoth lying in a small lake. The giant didn't attack me at all, and I was wondering if he's maybe part of a quest?

no hes not, but hes pretty cool, huh.

He might me related to the lone giant wandering Blackreach.

Since I'm new an cant post links
Google
The mountain goats - song for lonely giants