it's the old subzero - the character bio says his first appearance was in MK1.

[SUB-ZERO Wrote: Well, if you read the ending of classic Suby in UMK/MKT.... You wouldn't be so sure that he is the original Sub-Zero from MK1

The ending never mattered as it was only a cameo appearence, they were only bothered about the moves and specials Classic Sub-Zero had, don't look so deep into it, Boon+Co never meant for so much insight into it, they just needed to make up a story for him as an excuse to use him.

If you want to look so deep into it, put it this way, there hasn't been a 3rd Sub-Zero mentioned before in previous games, there's only 2.

Yeah, Sub-Zero6 *elder brother* is dead..sad stuff(said to be killed by Scorpion). I don't like Sub-Zero7's ALT-costume..specifically 'the bald head-ponytail combo w/plain mask'look...

~Sub-Zero5 wasn't in any of the games..he was "the brothers" father..Some German site had "the story"..it was pretty intresting.
PS: I love Sub-Zero's 1st fatality!

I think the older SZ isn't dead... So many characters died in MK history, but in next episodes they still lived....

Not only did Sub-Zero's Konquest storyline stated that the older Sub-Zero died by Scorpion's hands, but the MK team officially confirmed older Sub-Zero's death during Fight Night. When it comes to certain other characters, it's about careful observations in the storylines and stuff like that. Not all characters who died in MK are necessarily going to come back.