an iguana (see Molina); might also be a two-headed rattlesnake (see maquizcoatl, which is said to be another name for the tetzauhcoatl).
Alonso de Molina:
tetzauhcoatl. basilisco. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 111r. col. 1. Thanks to Joe Campbell for providing the transcription.