Kommentar zu Divrej Hajamim I 3:3

the sixth, Ithream, to Eglah his wife [The] Midrash [asks]: Was she his only wife? But she bellowed like a heifer when she bore him, and that she was beloved to him like a heifer [and was affectionately called Eglah, a heifer], like (Judg. 14:18): “If you had not plowed with my heifer,” and because of the endearment of Michal, he calls her “his wife.”