Breeding of mounts (currently horses).
Horse breeding calculates the tier of a horse and its look. While the tier is quite well defined, the color pattern allows a lot more variety.
How to obtain a Filly
Breeding your own Horses
Breeding with the Horses of other Players
Exchanging Breeding Pairs
Horse Tier Calculation
The tier of the horse plus its look determine the base values of the horse. Usually higher tiers mean higher base values for speed, agility, acceleration and breaking speed - but that rule is not 100% valid. Some lower tier horses have higher values than higher tier horses. What base attribute values a horse has is predicted by the look of the horse.
Horse Looks Calculation
The horse look is determined by a red-white-black code each look has been assigned. If you bread two horses, the values of both parents are summed up. Afterwards the RWB values of the filly are randomly generated until at least one of the values is the same or below that of the parents sum.
For example breeding parents with RWB code of 1-3-0 and 1-2-3 gives a sum of 2-5-3. Thus any filly with an 'R' value of </=2 or 'W' value of </=5 or 'B' value of </=3 or combinations of those is possible. For example 2-6-4 would be valid because 'R' is the same or below the parents 'R' sum. 1-1-1 would be valid as well - but 3-6-4 would not be valid.
Pattern code values of '0' are ignored, so breeding two 1-0-0 which has a sum of 2-0-0 will not allow a 4-x-0 but only a 1-x-x or a 2-x-x.
Purebred pattern have values of 1. But even if you breed two 1-0-0 together, you can still have anything from 1-0-0 to 2-0-0 to 1-x-x so forcing purebreds by breeding the parents is not possible.
There is an odd connection between look and tier of the horse which means every look belongs to one tier. So you can look at a horse and know it's tier if you have learned which belongs where. That impacts the ability to breed higher tiers as a filly of valid RWB code according to the parents code sum can have the "wrong" looks code for the tier of the parents. Thus, if you breed two tier 7 horses together but the RWB value of the filly is not a T7 one you will receive a T6 horse.
A calculator for horse look options is available here (you need to log into gmail and go to file -> make copy to have it show the drop down menu for breed pair selection).