Fallen Empires mount harm

While gang harm converts with no issue, vehicle harm and harm to mounts don't convert.

How do you handle it? Are mounts just part of the gang? Do they modify gang harm or increase gang size?

Maybe handle it as part of the fiction? Most of the Dark Ages sources that Fallen Empires draws on don't really focus on killing horses, although they do get killed now and then. 
Also, need to distinguish between mounted gangs that get off their horses to fight battles, which is most common, and actual cavalry.

EDIT: Hidden in the description for Sword-master is a description of the benefits of fighting from horseback: +1 Harm, +1 armor against non-mounted enemies, mounted enemies get no advantage against you. 
I would think those same benefits would apply to mounted gangs fighting against gangs on foot, but who knows? If you do go with this rule, it would be a good idea for it not to apply, or not apply in full, to infantry in defensive positions or shield-walls. 
I would probably treat mounted as a tag that can be applied to gangs, and which can be lost if somebody steals their horses or, for example, if they're besieged and forced to eat them all to survive.
                                                                                 

First of all should make it very clear that there are different types of horses. Generally speaking there are draft horses, riding horses and war horses. Only the latter can be made to actually carry a person into battle, because it goes against everything the animal wants to do. Assuming you have enough war horses to form a calvary, I would count that gang as if they are one gang size larger. If the gang should be medium and they want them to be calvary, give it to them but the gang size should start small instead. That's my take on it anyway.

When dealing "harm" to horses, I wouldn't bother. When a biker gang attacks another group you in AW you shouldn't be so zoomed in so far that you're tracking each bikes damage. The same goes for horses. If the suffer heavy casualties, then shit, they have a bunch of dead or injured horses as well. If the horses are hurt, it might take time to get tame/train/purchase/recover the horses.