When did cohors equitata milliaria come into existence?

[Condottiero Magno]

Cavalryman were in turmae of 30-32, not centuries of 8...

Well if you say so. But of interest, four centuries of eight cavalrymen does make 32.

There is no such thing as a century of eight...

Even if there was such a thing, 120 isn't divisible by 32 into an even number. The decurion might've been supernumerary, like the centurion, so he would've been the 31st man in a turma and 124 in an ala of 4 turmae or 128 or 132, if the two subordinates are also extras.