it says here that outcrosses can be stallions or mares.
'I have a registered Knabstrupper mare that has been inspected and approved for breeding. Must I breed her to a Knabstrupper stallion in order to register the foal?'
No. A registered and approved Knabstrupper mare can be bred to either an approved Knabstrupper stallion or to a stallion registered and approved with an acceptable "outcross" registry. Acceptable outcross registries are listed in the Breeding Rules.