When buying from the sales ring you are randomly allocated a horse from a sire and a mare in the sire and mare pools.

There are 300 sires and 200 mares available at any one time.

The pool of sires and mares change from time to time and at the time of buying the sire and mare will be picked from those available at the time.

Creates are created by historical sires and mares of the site

There is a line of thought that "Creates" are not as good as sales ring horses. This is pure conjecture and may or may not be true.