Of course, I've travelled the deepest Internet and met creatures most cannot even imagine in their wildest dreams. I can also speak 13375p34|<, swedish and other strange languages. And your avatar is scary.
Ahem, if the thief is known and famous, he'll get thrown into the dungeons won't he? Tough yeah, of course they won't send some random peasants against a warlord who decided to get a new horse from an enemy town, but he's disguised right? Though then again, if he's known enough, there propably is a bounty on his head already and thus there might be illustrations that more or less represent the original. Might just as well be that no one has lived to tell and thus they don't have an accurate image.
But the point is, yeah, the worse your rating with the said faction the higher level units are going to be sent against you. But should you happen to do your stealing (well maybe not for horses unless you slice them to small pieces and then assemble them again outside the city, but hey, how about stealing some salt from Zendar and selling it in the salt mine) undetected every time, you won't have a negative status and you might just as well be the best friends with a count and then tada - you put your mask on and go steal his horses.