Tepeyollotl, heart of the mountain, is a jaguar aspect of Tezcatlipoca, the smoking mirror god whose many faces the Mexica read as distinct enough to receive separate names and separate ritual attention while remaining, at root, manifestations of one another. His name and iconography tie him to caves and to the interiors of mountains, the places Nahua cosmology treated as entrances to the underworld and as the source of echoes, which Sahagun's informants describe as understood literally as Tepeyollotl's own voice answering from within the rock.
The jaguar form carries the animal's standing as the most powerful predator known to Mexica experience, an association shared with Tezcatlipoca's own jaguar manifestations and with the elite warrior order of the jaguar knights, though Tepeyollotl himself is a calendrical and cosmological figure rather than a patron specifically of that military order. He governs, in the reckoning the codices assign him, the trecena opening on 1 Mazatl, the deer, a placement that pairs the mountain jaguar with a prey animal in a single thirteen day week, and this trecena patronage is a separate role from his standing as the eighth of the nine Lords of the Night, the two systems of trecena patrons and night lords running alongside one another rather than assigning the same deity the same duty in both.
Sources Florentine Codex: General History of the Things of New SpainBernardino de Sahagun; translated by Arthur J. O. Anderson and Charles E. Dibble with Cycles of Time and Meaning in the Mexican Books of FateElizabeth Hill Boone