"Rock"? "MegaMan"? Or maybe just plain "Mega". Which is the true name of Dr. Light Number 001? Actually, I did some digging into the matter and found something interesting…
Did You Know?™ Megaman is never called "Rock" in any of the English-language versions of any Mega Man video game created or published by Capcom Co, Ltd. Not once. No, the English use of this name actually comes from… get this… the Ruby-Spears cartoon.
In the first episode which aired in September of 1994 - appropriately titled "The Beginning" - as Dr. Light prepares to activate his newest creation for the first time, he says the following:
"I'll call him Rock, my new robot prototype."
This is reinforced when Rock, himself, says shortly upon waking:
"Hi! My name's Rock! Who are you?"
As far as I'm aware, this is the first time Megaman is ever called anything other than "Megaman" (sometimes spaced, sometimes not) in any English media and quite possibly the only time, too. I say this because later, in 2002, Mega Man Powered Up was released worldwide and, despite having the extensive backstory for Megaman in it… for some reason, this bit of dialogue happens:
"A helper robot by the name of "Mega" lived peacefully […]"
Yeah. "Mega". They called him "Mega" in this game. Why? I don't know. However, it might be because of something that 1998 English release of Mega Man Legends did…
After defeating the Bonne robot named "Bruno", Tiesel Bonne asks who Rock is. Guess what he says.
"Me? My name is... MegaMan! MegaMan Volnutt!"
So, that's... quirky. It would be further expanded on in Mega Man Legends 2, for better or worse, in Barrell Caskett's Files - specifically, in "MegaMan's Discovery II":
"I decided to call the baby I found MegaMan. Actually, it wasn't really my idea... It was my granddaughter, Roll, who chose that name, after one of her favorite video game characters. I tried to get her to change her mind, but she insisted that should be his name."
Thanks, Roll. I can't imagine your "baby brother" getting teased for having such a flaky name or anything, no siree.
So, there you go. The US name of "Rock" started in the Ruby-Spears cartoon, was semi-retconned in a different timeline series, and finally ret-conned for real in a remake of the first game.