Apollodorus Library 3.13.5

[5] Afterwards he married Thetis, daughter of Nereus, ¹ for whose hand Zeus and Poseidon had been rivals; but when Themis prophesied that the son born of Thetis would be mightier than his father, they withdrew. ² But some say that when Zeus was bent on gratifying his passion for her, Prometheus declared that the son borne to him by her would be lord of heaven; ³ and others affirm that Thetis would not consort with Zeus because she had been brought up by Hera, and that Zeus in anger would marry her to a mortal. ⁴ Chiron, therefore, having advised Peleus to seize her and hold her fast in spite of her shape-shifting, he watched his chance and carried her off, and though she turned, now into fire, now into water, and now into a beast, he did not let her go till he saw that she had resumed her former shape. ⁵ And he married her on Pelion, and there the gods celebrated the marriage with feast and song. ⁶ And Chiron gave Peleus an ashen spear, ⁷ and Poseidon gave him horses, Balius and Xanthus, and these were immortal. ⁸