Mysteria Matris Oblitae

From the scenario:

"Turning up at the University of Mexico City in the summer of 1929 is a truly odd photograph received by postal courier from the town of Tula, Mexico. This photograph shows a bizarre creature surrounded by Mexicans and Indians. The beast, a pale misshapen alien thing, obviously succumbed to some sort of gunfire which wracked and punctured its odd crustacean-like body. It is clearly dead, and the locals show no fear in their expressions, only awe towards the beast which is approximately the same size as a medium-sized dog."