I came across this blog post on Jarilo that made some further details on associations that I made in my previous post regarding Jarilo (or Yarilo) and Apollon. the relationship with his twin sister, though different a bit from Apollon’s relationship with Artemis which though had tensions was not a marital relationship (though Roman interpretation seems to bring some of the romantic tension between the twins in Ovid’s interpretation of the story of Orion. However, Artemis bears a lot of similarity to the twin of Jarilo, so it may make a case why IF Apollon originated as Jarilo that when his cult entered Hellas with the Dorics that Artemis (whom most agree originally did not have a twin) was twinned with Artemis who represented similar forces as Apollon as acting twin parallels in the cosmos and nature. Artemis assumed the position of his twin which has become part of her identity since archaic worship of the twins together. This identity is now inseparable from her, but this would be a reasonable origin of her twin relationship if Apollon did indeed arise from Jarilo….and though Apollon is not strictly speaking a vegetation deity (vegetation deities being characterized as dying with the crops…though I haven’t seen anything that says Jarilo *dies*), he is only present during the season of growth and that goes to Hyperboreia…a far land where migrating swans go that is characterized as a garden of the blessed and shares a lot of similarities to the Slavic underworld! Additionally, we find similarities in his role as a herder, and his return to the world corresponds to the Doric/Theban/Delphic birth of Apollon in February. So I highly recommend reading this!
On a related note…I have found an image called the welcoming of Jarilo in which the god appears to be almost like a centaur but goat rather than horse, with goat horns…may this be behind the goat horns of the Doric Apollon?