No. There will be no horses (or other mounts) in Book II. To be fair, we tried...we spent 3 weeks in February experimenting with adding horses to the engine and it just requires waaaaay too much modification. Things we had to consider: what to do with horses in towns and dungeons, horses in combat, how do traps/disease/poison effect horses, horses in regards to boats and portals, how player encumbrance effects horses (and a horse's carrying capacity in general), food/water requirements (if player needs to eat, shouldn't their horse as well?), not to mention tons of additional animation needed to put horses in the game. Seriously, horses and mounts need to be worked into an engine at the development stage for it to work properly- it's not something that can be easily tossed into an already developed engine.
I do promise that it will be considered for any future game we do outside the original Eschalon trilogy.