A.23.1- The ability is written as "Horsemanship". [D'Angelo 2000/06/05]
A.23.2- A creature without Horsemanship cannot block a creature which has Horsemanship. [CompRules 2001/07/23 - 502.17b]
A.23.3- Horsemanship is a non-cumulative ability. Having Horsemanship more than once gives no additional effect. See Rule A.1.2. [CompRules 2001/07/23 - 502.17c]
A.23.4- Horsemanship is a static ability. See Rule A.3. [CompRules 2001/07/23 - 502.17a]
A.23.5- Horsemanship is an evasion ability (see Rule A.15), so it is cumulative with other evasion abilities. [CompRules 2001/07/23 - 502.17a]
A.23.Ruling.1- A creature with Horsemanship can block a creature without Horsemanship. [CompRules 2001/07/23 - 502.17b]
Note- Horsemanship has only been used in the Portal: Three Kingdoms expansion.