This is a character a friend of mine and I designed together. This is Prince Gaius Evergreen. Image was generated using Stable Diffusion, and PonyDiffusion_v4 checkpoint

Gaius Evergreen was the Prince of Nature, creating and growing the ancient Evergreen forest. Elder brother to the Princesses Celestia and Luna, Gaius ruled Equestria with his sisters for a few centuries.

When Discord took over, one of the things he did was corrupt Gaius, feeling that the male Alicorn was the greatest threat to him. In his corrupted state, Gaius lost his mind, and began to generate fierce storms across most of Equestria, which only empowered Discord through chaos. Eventually, Luna and Celestia were forced to put a stop to Gaius' madness. They entombed him in Crystal, and buried him within the earth in the middle of his forest, the sisters thinking that would be where he would want to be buried.

With Gaius' magic seeping out through the crystal and into the earth around him, leeching, unchecked into the once peaceful Evergreen Forest, it began to grow wild and untamable. After some time, the original name was lost, and it became known as the Everfree Forest.

Centuries and Millennia passed, and eventually, everypony forgot that there had once been three siblings upon the throne, not just two. Even Celestia and Luna forgot about their long-lost brother as the years passed. Gaius wasn't even a footnote in Ancient Equestrian History by the time Luna became Nightmare Moon, and when she returned and was reformed, there was no historical trace of their brother to be found anywhere within Equestria, except for a very few, very dedicated followers, who have spent generations trying to find their lost Prince.