PK5: Unobtainable Ghetsis's Kyurem ᅟᅟᅟ

Pokemon Black & White introduced a pair of main title games,
where the player could not catch the box mascot Legendary until past the first Elite Four challenge,
and Pokemon Black 2 & White 2 took that idea to a whole new level.

Featured on the boxes of Pokemon Black 2 & White 2, is the same Pokemon, but in 2 different formes: Black Kyurem and White Kyurem.
Their introduction may not have been the first canonical Pokemon fusion ^{(slowpokeXshellder anyone?)},
but it certainly was the first for Legendary Pokemon, and they were uncatchable in that state. (normally)

The antagonist that continued to plague Unova, Ghetsis, stole the DNA Splicers,
tricked his foster son N to appear with the legendary Pokemon of Unova,
AND managed to steal N's Pokemon Zekrom^B2/Reshiram^W2 and got Kyurem to fuse together with it.
(the cutscene looked like a dog toying with it's prey, but I digress)

This page hosts the Black Kyurem and White Kyurem, stolen from Ghetsis (karma, lol), by @jojo12100

If you didn't have a Fused Kyurem, these cannot be un-fused;
The separation depends on the the state of the DNA Splicers - which there's 2, hence why it can have different description.
Also depends if there is anything in the Kyurem Fused Slot in your save. (that's how they retain the Zekrom/Reshiram used for fusion)

As per any Black Kyurem/White Kyurem, it cannot be traded, and cannot be transferred to later generations, when it is in the fused state.
 

Original Posts:
https://projectpokemon.org/forums/forums/topic/39815-unreleased-prototype-pokemon/?do=findComment&comment=209228

https://projectpokemon.org/forums/forums/topic/39815-unreleased-prototype-pokemon/?do=findComment&comment=209338

https://projectpokemon.org/forums/forums/topic/39815-unreleased-prototype-pokemon/?do=findComment&comment=209351