This is some wonderful gift art from a while back by  MintyCandy not drawn by me.

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It started as just a tickle for Rillian as he trotted down the path through the forest. He paused in mid step, raising one hoof as he glanced behind himself. The fur on his back stood on end, even the hairs leading down his hind legs to his hooves were raised with anticipation. No pair of predator eyes were there to greet his gaze, nothing at all in fact, as far as he could see he was as alone as he was when he first began his journey. He turned in a circle looking up and down and around for any sign of the presence that he still felt, yet still there was nothing.

A sense of inexplicable dread began to well up in his stomach. Seeing nothing felt somehow worse than spotting a danger, and as he turned back to the path ahead his pace quickened. The feeling didn't go away, in fact he could feel a similar tickling as he hurried forward. It was like a gentle breeze blowing over his hind legs. He kept glancing to see if a massive monster was breathing at his heels, but still nothing. That same breeze seemed to shift from his hind legs, up his back, and right over his head when panic set in. If he couldn't see it but could feel something, the breathing must be from an invisible creature!

He took off at a gallop, pushing his pace fast enough that he was sure there would be a change in feeling over his fur. It lingered though, and seemed to become even more firm on his head. It was like a breeze was becoming a weight that grew heavier and heavier. His running slowed as it became clear his pace had no effect on the situation. He felt...tired, like he had been running for thirty minutes instead of thirty seconds, but that made no sense. He hadn't been pushing himself at all, and yet he already felt like he might not make the rest of the journey through the forest. The weight over his head seemed to grow and ooze down his head and around his neck, applying pressure and making it hard to breathe.

"No...no I have to keep..." he sputtered as he tried to push himself further down the trail. He staggered, stumbled, and his forelegs abruptly buckled under his weight as he flopped to the ground. "...so sleepy, how am I...I need help. Somebody, please he-URK!" His call was cut off with his airways as thick unseen scales tightened. He whimpered as it loosened just enough to let him take in a gasp of breath, feeling the unseen length curling under his midsection to further weigh down his body. He tried to get up, but he was so weak he could hardly even put up a fight.

A soft hissing tickled the pony's ear as the snake finally made its presence more clear. He couldn't crane his neck and see it, but he could feel the body swelling and thickening and lengthening around his body. Every inch it grew just made Rillian more lethargic, and in his helpless state he wondered if it used to be so tiny and light it would feel like just the tickle of a slight breeze. He moaned in the only words he had left to ask for mercy, but the snake's tail just kept lengthening as it curl loop after loop around his twitching and wiggling body. Soon he was swaddled up, only his head uncovered as the creature siphoned his energy, leaving him limp and barely conscious as the soft hissing lulled him into a deep deep sleep...