Thaumcraft offers several new mobs:
Angry zombies are larger, more aggressive zombies with red eyes. They appear occasionally among ordinary zombie spawns, but more often in magical areas. Along with the usual drops, they often drop Zombie Brains, which have magical uses.
Furious Zombies are even tougher, and appear to grow larger when they are hurt. They are spawned directly by Sinister nodes, similarly to a monster spawner, and with the usual limitations including light levels.
These glowing orbs through air and water, trailing sparks behind them. They spawn in magical biomes and in the Nether, If killed, they will drop "ethereal essence", a key resource for advanced magical work. Some of them may attack with lightning from a distance, but they are easily avoided and rarely chase the player. Wisp spawners (producing only hostile wisps) are found in the rare Obsidian Altars, found atop mountains.
These demi-humanoids spawn in magical biomes (including Tainted Lands, though the results aren't pretty). They come in three varieties -- most common are Foragers, but there are also Stalkers and Thaumaturges. They rush to pick up any loose items nearby, and can be convinced to trade for items containing Lucrum. (It is rumored that the individual types also have other favorite aspects.) Trading with Pech can provide a variety of magical items, some of which are rare or unobtainable elsewhere.
If a Pech is attacked, they and their friends will attack the player (much like wolves), but if the player escapes, the Pech will calm down after a few minutes.
Tainted lands convert ordinary mobs, including "farm animals", into horrible tainted versions. They also feature a couple of special monsters, the disturbing Tainticle and the Taint Swarms produced by Swarm Spawners. All tainted monsters inflict the Taint status effect with their attacks, and will attack any untainted monsters or players nearby.
Found in Tainted Lands and in the wake of magical accidents, these are far more aggressive than ordinary slimes. Like ordinary slimes, when killed they break into smaller versions of themselves, but thaumic slimes can also merge with each other.
Found in the Nether, these swoop at the player, filling the air with flame and eventually exploding. The blast does little damage, but delivers a startling amount of knockback. If you can find them among the flames, they are easily killed.
These intimidating creatures pursue those who delve too deeply into forbidden knowledge, or who interfere with certain unholy shrines. They appear as a set of ornate robes apparently floating over the ground, with no feet, and bring with them a mystical fog blanketing a wide area. However, our world is as hostile to them as theirs to us, and in our familiar worlds they are visibly transparent. Only in their home, the fabled Outer Lands, do they achieve full solidity and power.