To install, copy into %AppData%/.minecraft/mods with ModLoader installed. If you don't know how to do that, then I suggest you try YouTube for install tuts since I'm not writing one out here.
Here is a screenshot of the Horseless Headless Horseman:
The Horseless Headless Horseman has almost the same behaviour as the TF2 one, minus the screaming that scares the player. He will choose the closest living thing near it as a target, until it's chased them for over 30 seconds, they are over 30 blocks away or die. At which point he will pick a new target. He drops the Horseless Headless Horseman's Headtaker, which is a very powerful sword/axe:
Here is a screenshot of the Monoculus:
The Monoculus is fairly basic currently, acting similar to a ghast but in the normal world. I plan to add the teleporting functions in future updates (leaving a portal to the nether, which is a good ready-made alternative to the tf2 underworld). Also will add the monoculus mask and maybe an item to shoot the eye projectiles.
The spawn rates on the monsters might be a bit low, or high, or maybe you have suggestions for situations where they should spawn (can't edit base classes, so has to be a new idea, not like lightning striking a skeleton (unless you can find an api for that)). Please post any ideas for the spawning