Jarilo, the ancient slavic god of prosperity/harvest/protection, was born the 10th son of the all powerful thunder god Perun. He was born on the night of the Slavic new year (or Great Night), but was quickly stolen from his cradle by Veles: god of the underworld, cunning sorcerer, and divine guardian of cattle. Lucky for young Jarilo, the slavic underworld was more of an eternally grassy meadow, instead of a frozen wasteland or gruesome inferno. The child god spent most of his time with his adoptive father’s enormous herd of cattle until one fateful spring when he returned (swam?) home.
With every step he brought renewed fertile soil, where plants would once again grow, and he proceeded to keep exploring until he finally met with another sentient being. By happenstance the lady he met was his own twin sister Marzanna (Morana), the goddess of nature, winter, and decay. Incestual relationship was a really popular thing for gods back in the day, so they soon fell in love and got married on the evening of the summer solstice.
This was great and all, but Jarilo is the god of fertility… and let us not forget that fertility gods aren’t renown for their lasting commitment to a single partner. Marzanna eventually found that her husband/brother was unfaithful, so she gathered the other gods (except Veles of course, he was busy with the cattle). Together they forced Jarilo back to the underworld via dismemberment/ritualistic sacrifice. After brutally murdering her own husband, poor Marzanna becomes understandably heartbroken. In her sadness she becomes cruel and wroth (this explained the dreadful winters of the region), but the cycle of death and rebirth allows Jarilo to once again meet, marry, and be dismembered by his sister once again each year.
It was believed that Jarilo would often take the form of a magnificent white horse, strong enough to travel interminably upon his arrival to the living world, so the horse had a spiritual attachment to prosperity and growth for the Slavic peoples. There were also two main celebrations that were performed to honor Jarilo. The Funeral of Morana took place in the late winter and primarily involved burning and/or drowning a female-shaped scarecrow after brutally tearing its dress off. Pysanky took place in the spring and focused on the decoration of eggs, which are symbolic to fertility and rebirth. The latter of these traditions was adopted by the Roman Catholic Church and integrated into the popular Christian holiday, Easter. I’m sure it was much less exciting without baskets full of chocolate and candy, but I’m glad the pope has no plans of adopting the other tradition (the whole event just sounds itchy, especially for the owner of the dress).