With this word John explains (again) how these horses can be so effective as to be able to kill a third of mankind; it is through their mouths, that is, their toxic, hellish teaching. By repeating the reference to “their mouths” John stresses the significance and the power of their teaching.
19 For the power of the horses is in their mouths and in their tails, for their tails are like serpents with heads, and by means of them they wound.