I think it is the Arabian Horse. I have seen many horse breeds that descended from an Arabian Horse. But to be safe, I would say I am wrong at the moment unless some people would support me.

Many different breeds have been used in crossbreeding to create new breeds throughout history. Its often hard to determine exactly what breeds were used to create new ones as there may not be records and thus we can only guess until genetic research is used to back up those theories. Arabians do seem to be in many, but there are also TBs and other draftish breeds and such.

These days for cross breeding to create so called "warmbloods" or "sport horses" you have all sorts of crosses ranging from the ordinary to the unbelievable.

So cross breeding is still done today, right? If my memory serves me right, there is a cross-breeding between a horse and a donkey.

What do you call then horses that are crossbred from 2 different horses that are already crossbreds?

A mule has a donkey father and a horse mother. The other way around produces a hinny.

I think that thoroughbreds are the most widly crossed breed, although I think that htey have arabian blood back somewhere in the pedigree. During hte industrial revolution is was shires and other huge breeds that were crosses to provide horses capable of providing the horsepower needed. (pun intended)

You might also see paint or appaloosa crosses in sport horses, as breeders try to add a bit of color to make their stock unique.