I was looking at the talk page of the Wikipedia Cybele article and a reference to Phaedrus 3:20 caught my eye. I thought this must be the fabulist, rather than the dialogue of Plato, and so it proved.
A translation of all the fables is at Gutenberg here. Apparently there is some question as to how to number the first fables of book IV, or whether they are at the end of book III. But here is what is clearly intended:
THE ASS AND THE PRIESTS OF CYBELE.
He who has been born to ill luck, not only passes an unhappy life, but even after death the cruel rigour of destiny pursues him.
The Galli, priests of Cybele, were in the habit, on their begging excursions, of leading about an Ass, to carry their burdens. When he was dead with fatigue and blows, his hide being stripped off, they made themselves tambourines therewith. Afterwards, on being asked by some one what they had done with their favourite, they answered in these words: “He fancied that after death he would rest in quiet; but see, dead as he is, fresh blows are heaped upon him.”
The notes on this are:
Priests of Cybele)—Ver. 4. During the Festival of Cybele, the Galli or eunuch-priests of the Goddess went about with an image of her seated on an ass, and beating a tambourine, for the purpose of making a collection to defray the expenses of the worship. They were called by the Greeks μητραγύρται, “Collectors for the Mother.” See the Fasti of Ovid, B. iv., l. 350, vol. i., p. 149, of Bohn’s Translation.Tambourines)—Ver. 7. “The tympana,” which were almost exactly similar to our tambourines, were covered with the skin of asses or of oxen, and were beaten with the hand or a small stick.
Not that this helps the perplexed Wikipedian, but it is a useful reference all the same.