Silvanus

they approached the big pony, watching their steps, trying to not disturb the quiet atmosphere of the place.
"Prince of the Forest?" ventured Reed Whistle. The big Pony turned around, no emotion could be read on his face.
"Prince? The Forest is no place for such things." he said, and turned around to continue with his work. "Come here, little ponies, and help me" Reed Whistle looked at the other and nodded. He couldn't help but noticed the lack of a crown, jewelry, or any sign that he was, in fact, Princess Luna cousin. "Name's Silvanus, by the way."

Silvanus, a cousin of the Princesses, is in charge of the lands, like they are in charge of the skies, and their other cousin Pontus being in charge of the waters.
Unlike his relatives, Silvanus likes the simple life, living in a hut in the center of the Everfree Forest, which no pony can locate unless he wants them to... or one of his relatives gives them the exact directions.
He's a rather powerful Earth Pony, able to feel the land, and communicate with animals and plants like no other pony can(Another reason for his lack of fhoofwear: He likes to feel the dirt under his hooves).
A long time ago, after Nightmare Moon appeared, he switched places with Celestia, giving her reign over Ponyville, and taking care of the EverFree Forest, as "since no ponies will be willing to live there anymore, the animals there would suffer". He takes care single hoofedly of the whole Forest, changes seasons and takes cares of the animals("The monsters need a place to live too").
He's patient and understating, but lack of interactions with other ponies has made him a little cranky and uncooperative, it takes the efforts of Bloomberg, Tinker Tailor, Reed Whistle, Helix Gear and a gift of Princess Luna to get him to tell the ponies how to help Bloomberg become a real pony.
He takes a special liking to them, and they promise to visit more often, and help him out.

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Yeah, he's my take on a Forest pony-god, if Celestia and Luna are mares(And Alicorns), then I would make the stuff balanced by having the ruler of the lands be a stallion Earth pony, while the god of water will be a stallion hippocampus.
Also, unlike his relatives, he's a more "hands-approach" kind of ruler, and actually despises the title of "Prince", asking everypony he meets to call him simply Silvanus, his given name.
I took some aspects of him from deers(coloration) and the Forest Spirit of Princess Mononoke, I was even going to give him antlers, but then he would look like a deer instead of a pony