From the game guide under "Breeding and Watching Stallions":
WB sire X WB dam = WB foal
TB sire X TB dam = TB foal
WB sire X TB dam = WB foal
TB sire X WB dam = SH foal
SH crossed with anything except ISH = SH foal
SH crossed with ISH = ISH foal
RID (sire or dam) x RID = RID
RID (sire or dam) x anything = ISH
ISH (sire or dam) x anything = ISH
KNN sire X KNN dam = KNN foal IF a filly has LP or a colt has LP and PATN. If a foal doesn't meet these criteria, it will be a SH
RID or ISH sire X KNN = ISH
KNN sire X RID or ISH dam = ISH
KNN sire X anything not KNN, ISH, or RID = SH
Any sire not KNN, ISH, or RID X KNN = SH
AA sire X AA dam = AA
Any AA x TB = AA
X sire with X dam = X
Any X with TB = X
PON x Anything = PON if the foal 14.2 hands or under
PON x Anything = SH if the foal is over 14.2 hands
Any cross that results in a SH normally will result in a PON if the foal is 14.2 hands or under