Probably from 'amats; of a strong color, i.e. Red (others fleet) -- bay.
see HEBREW 'amats
H554. amots
[אָמֹץ] adjective strong, only plural אֲמֻצִּים of horses Zechariah 6:3, 7 (in 6:7 perhaps read אֲדֻמִּים, compare 6:2, & Hi; see another view in LagBN 29).