tick (see Molina), perhaps especially the ticks on deer, given that mazatl means "deer" (SW)
Alonso de Molina:
maza atemitl. garrapata. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 50r. col. 1. Thanks to Joe Campbell for providing the transcription.