I"ve had several recent encounters where a Legendary enemy was almost dead, then after the "kill shot" gets up with full health. Is this a normal part of the legendary enemy? I also had a legendary deathclaw, whose health was down to about 1/3, and a message popped up that said something along the lines of "this enemy has mutated" and he also had full health again. Is this explained somewhere?

Never explained really, but it isn't a bug, its a proper mechanic, once a legendary enemy reaches a health threshold they regain full health once, most likely as a mechanic to make them more lasting and dangerous.

As mentioned they mutate, and gain some unique trait during their "second wind."

Several I've encountered did this second wind thing and were glowing with wispy red flame. There was a mirelurk razorclaw that did this, and a Supermutant Butcher, and I hit two Ghoul Reavers in one location who did it, along with a third in the same location who didn't. Maybe I took him down too fast for the second wind to kick in, though.

I've had it happen a couple of times now. The first time a Super Mutant Butcher mutated and was bigger and badder was a real WTF moment in the best possible way.

I think this just happens at random. I've fought many legendaries, but not a single one has mutated yet. Then again, I keep restarting before even hitting level 25.

Yes, I am sure it is a random chance for it to occur once their health drops to a certain level. I have fought several Legendary enemies but only one of them mutated during combat, and the combat was not quick for any of them (well, not super quick, anyway... ample time for them to mutate if it was constant).

It's just something to keep you on your toes and earn the reward of the unique items they carry.

Haha Skyrim all over again.

I've only had 3 legendary fights but they've mutated every time. One if them had the creature mutate after my first pathetic pistol shot. Glad I got it out of the way at te start.

Kill them quick enough & they don't have a chance to mutate. Then again that's only happened once to me - that could've been the bug, idk.

The best, best, best way to kill something legendary, is to get the syringe gun & use the lock joint ammo - before it even knows you exist / change to mutate (unless it's immune obviously). The enemy becomes such an easy target

Either way, are they legendary because people wrote books about them? 'Legendary' doesn't seem to be the right word. Oh well.

I've killed a Legendary Raider by doing a double-tap headshot from stealth. First hit took out his life bar, and the mutation message appeared, and the orange flames just barely started around his body ... then less than a second later the second shot hit, which was still a sneak critical since I was only in Caution. The mutation message didn't even have time to fully fade-in onscreen. Legendary my butt.

Legendaries are enemies that have a http://fallout.wikia.com/wiki/Fallout_4_creatures#Legendary_Enemies depending on your difficulty level.

They are guaranteed a legendary drop, but they are incredibly difficult to defeat, not only because of their higher base stats, but also because of their mutations. Mutation chance has a correlation with how much health they're missing.

I've heard that they can mutate multiple times, can someone confirm this?

I've fought over a dozen of them and have 50 hours in the game. I've never seen one mutate twice.

I have had 2 legendary enemies drop nothing... The other 40+ have dropped as intended.

The best was when a mirelurk dropped a legendary pool stick.. I guess mirelurks play pool in thier spare time.

Haven't noticed any mutation yet (there's a possibility I wasn't paying attention), but most have regenerated health if I didn't kill them fast enough.