Poseidon travels across the sea on his chariot to the Greek camp at Troy. As he travels the sea creatures play beneath him. Κήτος (sea creature) is the word from which we get cetacean, the marine order which includes whales, dolphins and porpoises but in Ancient Greek can refer to any large fish or sea monster as well as whales and dolphins.

ἔνθ᾽ ἐλθὼν ὑπ᾽ ὄχεσφι τιτύσκετο χαλκόποδ᾽ ἵππω
ὠκυπέτα χρυσέῃσιν ἐθείρῃσιν κομόωντε,
χρυσὸν δ᾽ αὐτὸς ἔδυνε περὶ χροΐ, γέντο δ᾽ ἱμάσθλην
χρυσείην εὔτυκτον, ἑοῦ δ᾽ ἐπεβήσετο δίφρου,
βῆ δ᾽ ἐλάαν ἐπὶ κύματ᾽: ἄταλλε δὲ κήτε᾽ ὑπ᾽ αὐτοῦ
πάντοθεν ἐκ κευθμῶν, οὐδ᾽ ἠγνοίησεν ἄνακτα:
γηθοσύνῃ δὲ θάλασσα διίστατο: τοὶ δὲ πέτοντο
ῥίμφα μάλ᾽, οὐδ᾽ ὑπένερθε διαίνετο χάλκεος ἄξων:
τὸν δ᾽ ἐς Ἀχαιῶν νῆας ἐΰσκαρθμοι φέρον ἵπποι.

He came here and put the two bronze-footed horses under his chariot swift-flying and proud with their golden manes. He dressed himself with gold…