Nahual

Nagual or Nahual (both pronounced [na'wal]) is a word used in the study of the religion, mythology, folklore and anthropology of Mesoamerican peoples and which is used with different definitions. It is related to the concept that certain persons are able to take on animal forms, which is present in many Mesoamerican cultures. It is also related to the equally common concept in Mesoamerican cultures of all persons having an animal companion to whom they are spiritually linked throughout their lives. The most common usage within Mesoamerican anthropology is to call Nagual a person who has a particularly strong animal companion and who uses this companion to cause harm to others, or who himself changes shape into animals in order to cause harm to others. The normal practice is to refer to the animal companion itself as a Tonal, following the Aztec practice.
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[Aztec] The Aztec protectors of mortals. They are created from the same stuff as mortal, and each person has a nahual who looks after him.