Magic Monsters usually have purple color and look very magical.
There are currently 29 monsters that utilize the Magic Element
As of the 7/16/2012 patch, all Magic monsters share the following qualities:
When battling a Magic-based monster in the wild, your most effective fighter will be of the Poison element, as Magic is weak against Poison, and Poison is strong against Magic.
This battle combination will force your attacks to have a 1.5x multiplier attached to them, while incoming attack damage will either be cut in half, or the opposing monster will use a Normal attack if it has one.