Z/X (ゼクス Zekusu), short for Zillions of enemy X, is the general term for the creatures that comes from the alternate futures.
Z/X Encyclopedia Entry
Z/X is an abbreviation of "Zillions of enemy X" (an incalculable number of unknown enemies), which is pronounced as "Zeks".
It is said that it was a code name given by the Central Intelligence Agency when a large number of abnormalities appeared from the gigantic Black Point that appeared suddenly in Washington D.C., but the truth is unknown.
Those abnormalities were connected to the Black Point which served as a gate for them, and are the inhabitants from fivedifferentalternatefuturerealities, and it became clear from contact with the Z/X that they understood human speech.
Moreover, based on the characteristics of their home realities, it was found out that there were at least 4 different tribes from each timeline. Taking advantage of their unique traits, they have all come to the present to prevent the time-line of their rivals from ever happening.
As they have been separated into five groups, they each seek to ensure the birth of their future, while making sure the chances of the other ones happening falls drastically.