SYLLIS was a Naiad-nymph of the town of Sikyon (southern Greece). She was loved by the god Apollon and bore him Zeuxippos, a king of Sikyon.
Zeuxippos' name means "bridging the horse" or "yoking the horse"--a horse being both a literal horse and a poetic name for a stream or river.
Perhaps a daughter of the River ASOPOS
ZEUKIPPOS (by Apollon) (Pausanias 2.6.7)
CLASSICAL LITERATURE QUOTES
Pausanias, Description of Greece 2. 6. 7 (trans. Jones) (Greek travelogue C2nd A.D.) :
"After Phaistos (Phaestus) [son of Herakles and king of Sikyon (Sicyon)] in obedience to an oracle migrated to Krete (Crete), the next king [of Sikyon] is said to have been Zeuxippos (Zeuxippus), the son of Apollon and the Nymphe Syllis. On the death of Zeuxippos, Agamemnon led an army against Sikyon."
- Pausanias, Description of Greece - Greek Travelogue C2nd A.D.