Adam Clarke on Mark 11:2

Whereon never man sat - No animal was allowed to be employed in sacred uses, even among the heathen, that had previously been used for any domestic or agricultural purpose; and those which had never been yoked were considered as sacred. See several proofs of this in the note on Num 19:2 (note), and add this from Ovid: -

Bos tibi, Phoebus ait, solis occurret in arvis,

Nullum passa jugum curvique immunis aratri

Met. lib. iii. v. 10

The Delphic oracles this answer give: -

Behold among the fields a lonely cow,

Unworn with yokes, unbroken to the plough.