They were as fed horses in the morning
Adulterers are compared to horses, because they are very salacious and lustful creatures; wherefore the Septuagint renders the word: "horses are become mad after the females"; or, "as horses mad after the females are they become"; and especially to such as are well kept and are fat, and who, having much food given them in the night, and being full in the morning, go forth neighing, as Kimchi observes; and are the more salacious in the morning, by being so well fed all night, as those persons were, as is expressed in the preceding verse; though some render the word (Mykvm) , translated "in the morning", (for which sense of it see ( Hosea 6:4 ) ) "drawing out" F21; that is, the genital member, as lascivious horses do. The word is difficult of interpretation. The Targum calls them field or wood horses; horses that run in fields and woods, and are very vicious and wanton:, everyone neighed after his neighbour's wife;
coveted and lusted after her, signified his lustful desires, and sought an opportunity to defile her. Neighing is a sign of lust, and keeps up the metaphor of the horse.