Mods/Thaumcraft/Mobs

Thaumcraft offers several new mobs:

Angry Zombies
These larger, more aggressive zombies spawn in Eerie biomes (produced by a sinister node). Along with the usual drops, they frequently drop Zombie Brains, which are useful for some advanced crafting. There are also Baby Angry Zombies.

Wisps
Strange and wondrous creatures which spawn in magical biomes, these appear as a glowing orb flying through air and water. Each has a magical aspect, which is hinted at by their color. If killed, they will drop "ethereal essence", which bears not only the wisp's aspect, but also the precious aspect Aurum. Wisps of "orderly" aspects are neutral, not attacking unless attacked, but those of "evil" aspects may attack unprovoked. Wisps attack with lightning from considerable range, but it doesn't do much damage, and Wisps do not pursue the player very well. Note that in magical biomes, wisps also spawn underground, where they are fairly well trapped and easily killed. Wisps are common in the Nether too.

Pech
These demi-humanoids spawn in magical biomes (including Taint, though the results aren't pretty). They come in several varieties -- most common are Foragers, but there are also Stalkers and Thaumaturges. They rush to pick up any loose items nearby, but they have a special lust for precious or magical items, and especially those with the Lucrum aspect. (It is rumored that the individual types also have other favorite aspects.) The most common and dispensable items with Lucrum are the golden armor from monster kills (and gold swords, once the Nether is in play). However, advanced mages can do well with phials of Lucrum essentia, and mana beans work as well (but are less valuable). Throwing a few such items at a Pech will eventually "befriend" it, marked by green sparkles. Then the Pech can be right-clicked to open a "trading" interface. Offering another precious item and clicking the "dice" button will get you up to four items, which may be valuable or junk. (Note that items the Pech picked up will also be in the mix.) You can continue trading for several rounds, but eventually the Pech will just take the item and close the interface. You then will need to offer a few more "freebies" before the Pech will be willing to trade with you again.

Tainted monsters
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 Swarm spiderlings.

Hellbats
Found in the Nether, these swoop at the player, filling the air with flame and eventually exploding in a fairly weak blast. If you can find them among the flames, they are easily killed.