Xiuhcoatls are the fire-serpents of Mesoamerica.  They could breathe fire and withstand the most scorching heat.  They are sometimes called Turquoise Serpents or Lightning Serpents, depending on the associated deity. Xiuhcoatls carry the sun across the sky. 
Connection to Aztec Deities
The connection of the Xiuhcoatls to other deities earned them several titles, including Lightning Serpents, Fire Serpents, and Turquoise Serpents.
Huitzilopochtli ('left-handed hummingbird') is the Aztec god of war. Xiuhcoatls aided the newborn god to defeat his siblings,  which range from just his sister, Coyolxauhqui ('she with copper bells upon her face') the moon goddess,  to his sister and one brother,  to Coyolxauhqui and all the Centzon Huitznauhtin, the four hundred deities of the Southern stars. In some myths, Coatlicue ('she of the serpent skirt') gave birth to Huitzilopochtli, and he sprang up fully formed, painted in blue, with the flaming fire-serpents, xiuhcoatls, ready in his hands. 
Tlaloc ('he who makes things sprout') is the Aztec rain god.  Art found in the Basin of Mexico, from the time between the Teotihuacan period (around the first century B.C.) to the Conquest (fifteenth century A.D.), presents images of Tlaloc holding Xiuhcoatls, regarded as lightning serpents.  This is how Xiuhcoatls have earned the title Lightning Serpents.
Huehueteotl ('old god') is the original name for the god of fire. Fire was the center of domestic and spiritual realms of Mesoamerican Life, so the fire deity was the first identified.  Sometimes this deity is called Huehueteotl-Xiuhtecuhtli to reflect the name later attribute to the god of fire. 
Xiuhtecuhtli ('Turquoise Lord' or 'Lord of Fire') is the later name of the Aztec god of fire. Xiuhtecuhtli has many titles include the Lord of Time, Lord of the Sun, Lord of Fire, Lord of the Central Hearth, Lord of the Creative and Sacred Fire of Life. 
To the Aztecs, Huehueteotl-Xiuhtecuhtli was an immensely complex deity, and his alter ego, or spiritual guardian, was the Xiuhcoatl.  Xiuhtecuhtli's nagual is Xiuhcoatl, the fire serpent, and artwork from the Aztec period depicts Xiuhtecuhtli with Xiuhcoatls.  Xiuhcoatls are the primary attributes of this deity, and it is why they have the title Turquoise Serpents. 
Xiuhcoatl as a Nagual
In Mesoamerican myth, each man has a nagual, a guardian spirit that takes the form of an animal.  The nagual presided over a man's fate, and each person would have a life-long contract with their own nagual. 
Some individuals have the power to transform into their nagual, and while a person and their nagual aren't interchangeable, it is believed that when a nagual dies, so does the person to whom the nagual belongs. 
Xiuhtecuhtli's nagual is Xiuhcoatl,  which links the fire serpents to this complex deity and his attributes, including the sun, time, fire, and hearth.
Xiuhcoatl in Aztec Culture
In 'Song of Xipe Totec Iouallauan,' the fire serpent Xiuhcoatl becomes the quetzal serpent. Perhaps this explains the connection with the rain god and Xiuhcoatls. Quetzal serpent is a symbol of the green, rainy season connected to the rain god, Tlaloc. 
- Macmillion Profiles
- Markman [Flayed] 81
- Markman [Flayed] 163
- Markman [Flayed] 165
- Markman [Flayed] 203
- Turner & Coulter 509
- Willis 240
- Willis 242
- Gibson 51
- Nicholson 41
- Nicholson 42
For more information on footnotes and references, please see the bibliography.