Am I senile?
Possibly — since the forum’s resident telepaths are apparently on holiday, you probably know that better (unless people call you ‘Sleepy Joe’, that is).
WHERE IS RATTLEGORE?!
In your collection… or not. It also might be in a time-limited event, if I’m not mistaken, at least for those who haven’t done it, as per the 25.0.1 Patch Notes, which I suspect might be the case for you.
There is no such thing in the game as of this writing.
Yeah, it shows. Or maybe cuss words, with which you display no such difficulty and which you avail yourself of so profusely, are all you have ever learnt? 
Strangely, he is in Felwood (second page, after the original final boss Banehollow), since Karazhan bosses have been infamously scattered about three other different zones instead. You might have to unlock that fight first, though.
Well, at least they say such self-criticism is actually healthy. However, I also doubt your skills with search engines: you know, you could type ‘Hearthstone Mercenaries Moroes’ into the one of your choice and probably find that answer at once — just something to consider next time.
Is that even an actual word, by the way? Not sure…