The earliest records of the name come from Apollonius Rhodius (Ap. Rhod. 2, 791) and from inscriptions on coins of Tios dated to the early 3^rd c. BC. Later, the name is mentioned by Arrian (Arr. Peripl. 13 (=19M), Pliny the Elder (Plin. NH 6, 4), on 2^nd c. coins of Flaviopolis, on the Tabula Peutingeriana (TP 9, 4), as well as by Stephanus of Byzantium, who quotes Menippus’ periplous (St. Byz. s. v. Ψύλλα).

Interestingly, one of the coin types represents Bendis on a chariot according to B. Head.

W. Tomaschek defines the name as Thracian, compares it with personal names such as Βιλληός, Βιλληνή from Epirus and Βίλλαρος from Sinope and cautiously suggests that it may derive from IE *bhul- “to gush, stream”. D. Detschew refers to the personal names suggested as comparanda by Tomaschek, adding Latin ones, but does not comment on the etymology. In his article on Anatolian water names, B. Rosenkrantz doubts that the IE root *bhel- “shining, white” ca be found among them, while noting that if Βιλλαῖος belongs to these names, it contains an e-stem in the root, unlike other European water names, which have an o-stem in the same root. J. Tishler defines the name as ‘definitely non-Greek’ and considers the ending to be influenced by the numerous river names with this suffix. However, he was not convinced about its Thracian origin – he includes it in the list of Thracian names, but with a question mark. Since there are no other proper names with the root Bil(l)- in Thrace, Tishler is probably right that there can be no certainty about the Thracian origin of Βίλλαιος. In fact, Thracian origin is suggested mainly due to the fact that a significant number of names from Bithynia have such one. This hypothesis is perhaps supported by the fact that the root Bel- (probably from IE *bhel-), from which several Thracian place and personal names are derived, also appears in the variant Bil-, for example in the personal name Βελισάριος/ Βιλισάριος. A settlement name Bilias in Thrace is mentioned by the Anonymous Geographer of Ravenna (GR 4, 6). Furthermore, the representation of Bendis on one of the coin types bearing an inscription Billaios, supports the Thracian origin of the name.