"El Toro" is played by famous film director Guillermo del Toro
One of the rings that Guillermo del Toro wears references Miskatonic University a fictional university located in Arkham Massachusetts created by H. P. Lovecraft.
This episode marks the 6th time Stephen Root and Richard Riehle have worked together, whether sharing screen time or, in this case, not. Most famously, they appeared as co-workers in the film Office Space (1999).