The original release of Skyrim Monster Mod contained some creatures illegally ported over from other commercial games, so it was removed from the Nexus.
Another modder, ironman5000, then came along and cleaned up the mod to remove the questionable content, and has re-released it as "Monster Mod".
There are currently two versions floating around the internet - the original version is still being made, with the questionable content still included, but is only available on the darker corners of the internet.
The clean and fully legal version by ironman5000 is available from the Nexus here: http://skyrim.nexusmods.com/mods/35631
The lore-friendly version (by me) is still available from http://skyrim.nexusmods.com/mods/16411, although I'm working on an update, but I've had reports that it works fine with either version of the main mod (either version must be installed first) - but may be missing a few creatures (only a few, out of the hundreds still in the mod) until I can get it updated.
As an alternative to Skyrim Monster Mod (or in addition to it), I'd suggest Skyrim Immersive Creatures - http://skyrim.nexusmods.com/mods/24913 - it also adds hundreds of new creatures, but unlike SkyMoMod, the Immersive Creatures will spawn in more sensible and likely places.