You know what, a couple times now, there has been an Orcish Dreadnought in The Monastic Cavern where I've never seen one spawn before. I've been playing this game for 7+ years now and I have been through that place more times than I can count and NEVER had I seen it where I saw it - in the east tunnel at around G-8/9, south of the small open spot where the 2 tunnels meet up again and there's an Orcish Champion (which has always been there). The Dreadnought isn't always there, but the first time I went there and it was, I didn't notice it until I heard the grunt noise the Orces make upon aggro and I thought "I'm level 90, what the heck is able to aggro me in this tunnel... oh there it is."
Being around for 7 years, this wouldn't have been unheard of back in the early years of the game because monsters didn't used to despawn once the people who aggroed them initially died or warped or whatever. They'd stop, turn around and slowly walk back to their spawn point which would leave them to aggro anyone in their wake (sorry if you remember this time, just inform the newer players). I have, now fond, but then not so fond, memories of massive trains of bats or even skeletons being brought to the Garlaige Citadel's zone and then all of it wiping out parties. There used to be 3 or 4 parties that would be on the first steps just inside gaining experience and every last one of them had to zone themselves or risk being crushed by the returning monsters. I also remember this being used by a higher level DRG back when I was still taking my first job, THF, to Lv75 and in Yhoator Jungle. This individual decided to pull a Tonberry all the way to the Yuhtunga Jungle zone (which, with the distance from the first Tonberry to that zone line, he did it intentionally I think).