Basic Creature Script
The Basic Creature Script (BCS) is a standardized script for creatures. The BCS was implemented in 1998. Every critter since then has been BCS and a large number of the older critters have been rewritten to it as well. Critters often have innate abilities which are not tied to standard combat and magic systems, and not necessarily subject to the same rules.
Creatures that are under the BCS typically behave properly when delimbed (no right arm means no spell casting, for example), are capable of leveling up when they successfully kill a player character, and typically do not have a single set level, but rather, a range of levels.
Non-BCS creatures can have unusual behavior, such as casting without a right arm or a severe head injury, or have some other unusual properties. Non-BCS creatures, due to their unusual code, tend to be poor choices for Animate Dead subjects.