As well as being populated by the various humanoid races, the Warhammer world also contains many species of animals, some of which have been tamed and trained to perform a useful function. These can be divided broadly into two groups - animals that can be ridden (e.g. horses, giant wolves, cold ones) and those that can't (e.g. warhounds, giant rats). These rules are intended to flesh-out the rules for mounted warriors from page 163 of the Mordheim rulebook.