cauallo mecatl.

(a loanword from Spanish)

cauallo mecatl.
Principal English Translation: 

the halter for a horse (see Molina)
(partially a loanword from Spanish; caballo, horse)

Orthographic Variants: 
caballo mecatl
Alonso de Molina: 

cauallo mecatl. cabestro.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 12v. col. 2. Thanks to Joe Campbell for providing the transcription.