Nahual vs Nahualli

[Roland Trevino]

 Can these words be used interchangeably?  I have seen them used so and i
wondered if this was correct.  From the dictionaries that i have it seems
that Nahual refers to the animal companion or twin and Nahualli means
"sorcerer or warlock."  I know that Nahual derives from the word Nahualli
but are their meanings really interchangeable or is this incorrect usage.
Also, is the word Nagual completely incorrect or merely a barstardization of
Nahual?

~Roland