Rook is the most effective counter to horn by a long-shot.
Yesterday I played as Rock against minion Vampire. So what I did ? I decided to take Hammer Slam, to kill his minions in 1 shoot (at least that was my idea
). First lvl of Hammer Slam of course didn't do much to Vampire's minions. But I was very suprised, when I finally reach 1300 dmg Hammer Slam, Vampire's minions got... ~1800 hp !! Beside this guy can control them, so when he sees Hammer Slam going down, he just run his minnions throught me and activate HoB. Well that wasnt bad, I could kill them (when HoB finished), but when HoB was active, minion swarm could dmg me for around 2000 hp before I killed them !! And still, those are MINIONS, so Vampire can easly "make" another ones when they dies !!
Some time ago people cried here how Regulus annoying is, because you don't even see him when he snipes. Well, believe me, this vampire almost whole game was sitting right at his towers, near the creep line (so he got his backups almost instantly !!). That was fortress game, so before I killed all his minions, every wave destroyed like 3-4k fortress hp. And when I was happy I finally killed those bloody minions, I saw another wave comming. So if Regulus is not interactive char, what do you think about fighting vs minions, which Generals can summon basically FOR FREE every 30 sec, and you get nothing for killing them ? Isn't that pointless fight ?
Well, the truth is I don't really get this Generals idea. Generals can summon minions, and still they have attack like Assassins. Well maybe QoT is really bad in attack power, but beside all other Generals have very nice attack, and still can use minions (minion priests at the beggining of game are VERY helpfull - you don't have to return to base every time you are low hp; minion archers can easly outrange towers, so are very good either, especially early game, before opponent gets towers hp regen). So where are Generals (except QoT) weak points ?

Yes, I know one of the Assassins (beast) has highest AP (especially early game), but beast can't heal himself, so you really can't compare basic DPS between beast and eg vampire (and any other classes, because eg TB offensive skills do the difference - table above only shows basic attack DPS, don't show skills). Anyway, Generals have very nice AP, they have defensive abilities (Oak shield, QoT shield, Sedna heal, Vampire heal), and still can use minions...