So I was exploring the woods of skyrim outside of solitude and I came across an abandoned shack with a dead guy in it thought he got attacked but there was no blood and he was lying on his bed with his arms crossed, he definitely dead cause when I put my cursor over him it said nord and I was able to search the dude. I robbed his house and then left.. A few meters ahead heading back to the main road I see a dog called Meeko he barks at me then runs to the dead dude shed then I realise that he's his dog I press talk to Meeko and he now follows me like a companion. He doesn't talk though he barks but u can tell him to go home... Later I encounter a bear with Meeko and Meeko attacks getting his butt whooped and damaging the bear a lot Meeko is nearly dead so doesn't attack and the bear doesn't attack I thought Meeko was dead it after I killed the bear Meekos health generated back to full straight away and he never dies i don't even need to fight he kicks ass well only recently he deafeted a giant, so not sure a glitch or just a really bas ass dog

I accidently killed meeko once so hes not really invincible but if your looking for a compainion who never dies get barbas and never finish the quest or if you have the dawnguard dlc get serana she only goes down but never dies even after you finish the main dawnguard quest.

Oh damn I forgot to mention that he doesn't die unless u attack luckily I saved before I went into the battle but yeah I forgot he only dies if u kill him

Any companion that is essential is technically an invincible companion. Mjoll, Frea, Serana, DBI are some of the better ones, well not Mjoll but you know what I mean.

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I don't have dawn guard dlc and I don't think Meeko was actually part of any quest but i don't actually know some could be right