No, you're thinking of the son of Darkseid, O'Ryan.

What DC comics icon originally appeared to have a nickname, only to have that established as his actual last name?

The flash?

Adam Strange?

No, wait, second guess: Sgt. Rock?

Nope.

I mean the character's legal name, not a superhero codename.

You mean, sort of like CuCr's irish name that people thought he just used when he was a highly respected, well-paid actor in adult films, Rod O'Steele. No one knew it was his actual name.

Anyhow, I have no clue what the answer is, so I'll toss out a guess.
Sinestro.

I ought to refresh before posting, especially when minding a 4 year old.

This is correct. The first story called him "the rock of Easy Co" but I believe did not give him a name.

The Matter Master once gave five Justice League members the heads of animals. Which five, and what animal heads did he give them? (No, I'm not making this up.)

Yeah, I remember this one. In the first part of the story, Matter Master's lawyer visits J'onn J'onzz, but J'onn won't back down, and Matter Master gives him a horse's head! It was really gross, it was in J'onn's bed, and J'onn woke up and realized the bed was all bloody, and then found the head, and then screamed and screamed.

Hang on. That wasn't JLA, that was the Godfather.

Good scene, though.

Here are my guesses:
Wonder Woman - bird head
Aquaman - dolphin head
Flash - cheetah head (because cheetahs are fast, get it, get it?)
Green Lantern - dog head
Martian Manhunter - gorilla head

--man, comics were awesome once, weren't they?
wyld

They sure were!

You got two of the five correct JLAers and none of the correct animal heads. I'll give you a hint: two of the five JLAers you name were not active members at the time this story took place.

Aquaman with a fish head
Batman with a rat head
Flash with a deer head
Elongated Man with a snake head
Green Arrow with a fly head.

That was the first appearance of the Golden Eagle.

I think Batman was a mouse rather than a rat, but that's close enough for jazz. All yours, JTR!

(Poor Barry was always getting transformed into stuff, wasn't he? In his adventures with the JLA he was given the head of a deer, the body of a snail, and had his legs fused into one appendage; and in his own title he was turned super-fat, old, two-dimensional, into a giant puppet, and into a living mirror, just to name a few off the top of my head! No wonder he's been hiding out in the Speed Force for twenty-odd years!)

Yo, JTR, make with the question already.

--wyld