Board Thread:General Discussion/@comment-32147440-20171202005054/@comment-1458758-20171217232650

Considering the vast number of sapient races stored in the Omnitrix, it seems likely that all Bens, regardless of universe, theoretically possess all the aliens.

Eg. we know Reboot Ben had Gax, we also know that Ben Prime had a Chimera Sui Generis in his Omnitrix, but didn't know he had it (The sample coming from somebody other than Vilgax)

While there are some unique cases like the Andromeda aliens, Crystalsapiens and Vladats, as others have pointed out, Reboot Ben and other alternate reality Bens have possessed aliens from Andromeda without having necessarily encountered them, so I'd say it's probably a safe bet that what was true for Prime Ben need not necessarily be the case for others.

Long story short, I work under the assumption that unless this series were to somehow produce over 1,000,913 alien species, we should naturally assume that the Omnitrix from any reality possesses all naturally occuring and artificial forms of sentient life encountered in the series. So not only the aliens used by other Bens, but aliens that we know exist, but haven't been used, like the Kraaho, Lewodans and Detrovites. Even species that wouldn't be expected, like Anodites and Naljians, I mean, if something like Alien X can exist in the Omnitrix, not to mention the various mineral based aliens, I don't see why any alien would be strictly excluded from the list.