Gone, But Not Forgotten: The Undeath of Caeneus, Pindar, fr. 166f / 147 f.

Although it does not mention the transformation aspect of the myth of Caeneus, this version does attest to his invulnerability. 

 Struck by the green pines, Caeneus broke the earth with his foot and disappeared.

ο δε χλωραϊς ελάταισι τυπείς ώχετο Καινεύς σχίσαις ορθω ποδι γαν

Caeneus vero virentibus abietibus percussus abiit, recto pede terra scisa.

--Pindar, fragment 45 (1821; modern number 166f/147f; attested in Schol. Apollon. Argon.I.61); Translated into Latin by Augustus Boeckhius (1821)

 Pindar is a famous Greek poet from Boeotia (modern Greece) known for his victory odes. These odes, for victors of Pythian, Nemean, and Olympic games, are rich in mythological imagery, and help us understand the relationships of the ancient Greeks to their cultural heritage and their understanding of the past.