/spawnmob is the spawning command. It's not region restricted, so you can /spawnmob pigzombie outside of the nether. If you want more than one, you can do like /spawnmob zombie 10. If you want more than ten, you have to edit the Essentials config.yml file and up the limit as it's hardcoded at 10 by default.
Well, I was not well aware that you were well aware heh. Also, I failed to notice you said "naturally" in the original post, my bad.
I was curious about this, so I fired up Eclipse and toyed with making a plugin to do 1. spawn pigmen in overworld and 2. spawn lots of mobs. I think in my test run I found I went a tad overboard; players would be slaughtered continuously if they got on my server right now, hehe:
Needs some tweaking. As it shows, I also have it spawning everything regardless of world it comes from. What I had it do was whenever a mob is spawned naturally or during chunk generation, it spawns 5 random mobs in close proximity to the new spawned mob.
Hmm. I can have it spawn more mobs during each single mob spawning, which would appear to have the same effect, but don't yet know how to adjust the actual spawn rate overall. Need to research that. The way I use works, but I want to figure out the other way so they appear at more random locations. I'm using the location of the newly spawned mob then adding a random amount of variance in X/Z to try to emulate that.