פירוש על עבודה זרה 45:6

The Talmud points out that the issue in that mishnah may not be the concerning for bestiality. It could be a concern lest the non-Jew had used the red heifer for work, for even the smallest load placed on the red heifer disqualifies her from being used in the purificatory ritual.
The “calf” referred to here is the calf used in the ceremony of breaking the neck and pouring the blood into the river to expiate for an unknown murder (Deuteronomy 21).