ירמיהו ה:ח פירוש: רש״י

Like armed stallions they would arise early Like these stallions who are armed with erect membra, every morning they would arise at daybreak.

they would neigh Heb. יצהלו. This too is an expression appropriate for horses. Comp. (8:16) “From the sound of the neighing (מצהלות) of his steeds,” an expression of shouting (esjodement in O.F.).