Usins was a pre-Christian Latvian god with several simultaneous purposes. His primary role was as driver of the solar chariot, not unlike the Greek Apollo. He had white, shining hair and a beard and his chariot was drawn by a pair of white horses. His chariot belonged to the sun goddess Saules, for whom Usins drove it across the sky.
This was the symbol of Usins:
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Except usually it's not crappy ASCII. The two E's are representative of Usins's horses. In some variations, the symbol would include a diamond below the rest of the thing, which was supposed to represent Usins in his chariot.
Usins was part of a complicated family tree. He was married to a dawn goddess, by whom he had twin sons and a whole pile of daughters. He was one of the Dieva Deli, meaning that he was one of the twin sons of the god Dievas, the Latvian form of the Lithuanian god Dievs. Both the Latvian and Lithuanian forms of the god were supreme, leader-of-the-gods types analagous to the Greco-Roman pairing, Zeus and Jupiter. The other twin was named Martins, a name which translates to the English name Marshall. Both names mean roughly "man who takes care of horses in the stable," which was the role that Martins played when the horses were locked up in the barn for the winter. During the summer, though, the horses were allowed to run relatively free, during which time they were thought to be ridden by the other twin, Usins.
On the first day of spring when the livestock were first let out of the barns for the year, the yearly festival of Usins would commence. We know for sure that horse racing was the primary draw to this festival, but the source I'm reading suggests speculatively that there may also have been other, more dramatic rituals as well. Based on the fact that the Norse and Celtic peoples usually had a winter god versus summer god duel which was played out by actors at the festival, the source states that the Latvians had some sort of dramatic presentation at the festival during which Usins would defeat his brother Martins. Afterward, the source goes on to speculate on the same basis that Usins would have been allowed to take a spring goddess in marriage afterward.
Usins was not only worshipped in festivals, though. His likeness, in the form of a painted head of a horse, called a Zergeliai, was usually placed on the roof of homes of newlyweds in order to gain favor with Usins. Also, there's his association with twins. In addition to being the father of a pair, he was also very similar to the twin horse-gods of other cultures, like the Asvinai of Latvia, the Asvins of Vedic India, or the Dioscuri of Greece. Based on all this evidence, then, it appears not unlikely to me that Usins might also have been a sort of human fertility god in some sense due to the special measures taken to gain his favor for newlyweds and his possible role as one himself every spring. Of course, that's mere speculation on my part.
In the course of researching this god, I found the text of a folk song about him, which I'll reproduce here:
I gave to Usins
What I promised:
A coal-black cock with scarlet comb
And every one of his claws.
I sacrificed a cock to Usins
On the morning of his day
So that he would herd my horses
All the summer long.
In Latvian:
Devu, devu Usinam
Ko es biju solijusi
Melnu gaili, sarkanse ksti
Ar visiem nadziniem
Usinam gaili cavu
Pasa svetku ritina
Lai tas man zirgus gana
So garo vasarinu
Sources: www.pagan.drak.net/wwcrew/deities/usins.html
http://www.geocities.com/cas111jd/balts/balts_usins.htm
http://www.angelfire.com/al2/LatvianStuff/SongsUsins.html
http://webhome.idirect.com/~upmanis/latsym.html