Among the many landmarks of the mystical topography of Colonus that we see described in this most remarkable passage, I highlight a rock that is called by the name of Thorikios Petros at line 1595 of Sophocles’ Oedipus at Colonus. The meaning of this place-name is complex. The noun petros means ‘rock’ or ‘stone’, while the adjective thorikios is derived from the verb thrōiskein, which means both ‘leap’ and ‘emit semen’. I quote here an antiquarian report about the relevant myth:
Others say that, in the vicinity of the rocks at Athenian Colonus [Kolōnos], he [Poseidon], falling asleep, had an emission of semen, and a horse named Skuphios came out, who is also named Skīrōnitēs.
Scholia for Lycophron 766
ἄλλοι δέ φασιν ὅτι καὶ περὶ τοὺς πέτρους τοῦ ἐν Ἀθήναις Κολωνοῦ καθευδήσας ἀπεσπέρμηνε καὶ ἵππος Σκύφιος ἐξῆλθεν ὁ καὶ Σκιρωνίτης λεγόμενος.
There are other versions of the myth that are tied to places other than Colonus. Here is an example:
Poseidon Petraios [‘of the rocks’] has a cult among the Thessalians ... because he, having fallen asleep at some rock, had an emission of semen; and the earth, receiving the semen, produced the first horse, whom they called Skuphios. ... And they say that there was a festival established in worship of Poseidon Petraios at the spot where the first horse leapt forth.
Scholia for Pindar Pythian 4.246