so I just got to Vranik and spoke to Erik and suddenly I notice I lost my horse?? It’s not active in inventory and I can’t whistle to call him! Is that a bug and I will need to reload some older save?
Well, so few scenes later I have my horse back like nothing happened :-] I think it might have been better if there was more explanation in the journal.
Yeah your horse just probably got bored and ran off 
Nah it was probably a bug, since you should be able to call your horse at all times. Even if you forget him somewhere. one time my horse got killed and i though thats it, im alone in the wild and have to walk hour to nearest village, but luckily i was pretty near bandit camp and they had horse.