I made a new world for this. Killed the first 5 humans that were right next to me at the spawn point. many hours later i have seen no more. Never saw a zombie. Same spawn rate as wolves probably worked in the last version of minecraft, but I haven't seen a wolf since the 1.8pre. Please increase the spawn rate, as it is i feel like I'm playing single player for no reason.
Hmm.. well I tamed a zombie and he followed me for a while but didn't feel like fighting anything. I restarted my client and he started attacking stuff, except he didn't follow me anymore. It still said he was tamed, but when I left him he didn't teleport to me.
Only mods I have installed are modloader and this.
I'm starting to think they are generated with the world. So you'll only see them in the first chunk you spawn in after creating a world. Also, if you leave without killing them, and then return, they're gone.
Edit: Nevermind that, they do spawn. It really is just rare. Rare to the point you may not see any for hours. Also they do despawn if you leave them, and they never respawn.
UPDATED!!!! GenuineNpc 3.2 v1.8.1
Fixed must bugs as of yet, mainly the despawning bug and the "tame but not yours" after saving bug.
Player suggestions ROCK!!!
Added: Command to stay or follow by right clicking them with anything but brains.
Added: Tame npcs now protect the player MUCH better. this will override your command of staying but only while protecting you.