Descended from the wild ponies of Kislev, imperial horses are suited to harsher climates.
The finest horses in the Old World, bred for their endurance and fiery temperament.
- Hide (forest)
The most basic mount available to a number of Heroes and Lords, the Warhorse frequently gets passed over in favor of stronger options later, as a point spent on it is still a point spent. However, it does have certain things to recommend it as a pick.
First, quick access--the Warhorse unlocks at level 4-7 for most classes that can get it, allowing you to mount up your Lord very early in a campaign. In Multiplayer, it is the cheapest option, which produces a similar result--you can gain a mounted Lord/Hero while saving money you can invest elsewhere in your army. Simply being on a mount has several advantages--the character's size rating increases to Large, which does make them vulnerable to spear-type weapons, but also keeps them from being bowled around by enemy Large units (which can easily lead to stun-lock and a quick death) and allows them to do the bowling against the enemy's man-sized units. All Mounts also offer a substantial increase to the unit's Charge Bonus, meaning they hit much harder and break more leadership on the charge.
Thus, the Warhorse serves as a good "budget" option for simply getting on a damn horse--it won't increase upkeep as much as later mounts, while still providing the advantages of cavalry. The other mounts are certainly better, and if you think you can manage until you level up to them (or can afford them, for multiplayer) then by all means, wait. But if the enemy is just begging to get stomped by cavalry and you can't wait, the Warhorse is there.
Something important to note is that AI faction leaders who have access to the Warhorse (Generals of the Empire primarily, which includes all of the Elector Counts) will almost always pick it up--and every other mount they have access to. This is likely because the mounts used to have to be picked up progressively, and the AI might not have been changed to recognize that this is no longer the case. The odds of an AI General of the Empire or Hero having a Warhorse unlocked is very high--this can be a reason to try and confederate them as quickly as possible if that is your goal, to prevent the Generals you will eventually get from having too many wasted points on duplicate mounts.