Central to the four-year journey each soul made through Mictlan was a specific, well documented practical belief: that a dog was needed to guide the deceased safely across a wide river encountered along the way, and that a dog of the wrong coloring, entirely white or entirely black according to different accounts, could not or would not perform this service. This belief gave concrete ritual shape to Mictlantecuhtli's underworld, since it meant that dogs, and specifically the hairless breed today called the Xoloitzcuintli, whose name honors Xolotl, the canine god associated with this same underworld crossing, were kept and, according to Spanish colonial chroniclers describing practices they witnessed or were told about, sometimes sacrificed and buried alongside their owners so that the animal's spirit would be ready to perform this guiding role in the afterlife.
The belief reflects a broader Mesoamerican pattern in which the journey to the realm of Mictlantecuhtli was imagined not as an instantaneous judgment but as an arduous, multi-year passage through a series of dangers, obsidian winds and hostile creatures are described across different colonial-era sources such as the Florentine Codex, though the sources are not perfectly consistent with one another on the exact number and order of these trials, a variation that likely reflects genuine regional or temporal differences in Mexica underworld cosmology rather than error in the surviving accounts. The dog-guide motif nonetheless remains the most consistently attested single detail of the journey, tying Mictlantecuhtli's grim, static rule over the dead to an active, ongoing practice among the living.