Powergirl is not kryptonian or even quite an alien, she’s the granddaughter of Arion, an atlantean sorcerer-king, meaning she’s closer to Aquaman and Captain Marvel in many aspects than the big blue cheese.

Superboy-Prime is not Kryptonian either, he’s, according to himself, a meta-human from one of the Earths destroyed in the original crisis.

Sodam Yat & Mon-el, as previously mentioned, are Daxamites, that are a whole different alien race that happens to share the power gimmick. It does get the question if Jor-el could not send his boy to Daxam instead.

Krypto comes from the artificial Krypton (that curiously, existed inside the Phantom Zone!) created by Brainiac 13, one of the villains of Worlds at War, as part of some obscurely convoluted plan to take down Superman and make a future universal domination army - it failed big time and (supposedly) it was all lost and destroyed. The dog survived because it was on Earth and had already become Kal’s pet and Lois’ watchdog (duh).

It has never been mentioned afaik, but it’s a somewhat logical possibility considering the rest - that Supergirl, the Phantom Zone criminals and the Kandorians are remnants or a remake of Brainiac 13’s fake Krypton, specially considering that a Brainiac figures proeminently in their purported backstory.

And now my logical processors are in need of serious cooldown of all this geekiness overload.

http://www.comicsalliance.com/2009/10/05/super-social-networking-comic-book-character-facebook-status-u/