From dictionary.com:
the·ri·an·throp·ic [theer-ee-an-throp-ik]
adjective
1.
being partly bestial and partly human in form.
2.
of or pertaining to deities conceived or represented in such form.
[from Greek thērion wild animal + anthrōpos man]
They may well be part of a Shamanic religious culture that existed in South America. Here's a pertinent documentary on a proposed relationship between this type of art and altered states of consciousness:
http://www.youtube.com/watch?v=0qgMFO0KU-II know these presentations are quite long, but they're definitely worth the watch for those who're interested in these subjects.