Dagmar
Defender of the Bunnies of Skyrim
Odd, I have never had an enemy kill him, ever. In my current game I have had him with me for over 200 hours and only recently left him at home. He was with me til level 45ish. I had killed him many times, until we figured out a fighting style. He has taken on both mammoths and giants and not died, even after taking multiple hits from both.
As a follower he's a protected NPC when he follows you, meaning he gets one free pass when he is close to death and enemies will ignore him as he recovers health (for human followers they drop to one knee during this phase but as a quadriped Meeko doesn't). He'll die if it happens a second time in the same fight and he can still die from splash damage or a stray arrow which is probably why you ended up killing him accidentally.
If you get swarmed, as you can in certain locations that's generally where he may die, not against a few mammoths and giants