Worms

Flail landed with a soft thud into the sand of the beach of the small lake. He had found this lake the week prior, and he intended to keep this place secret. He dropped his pack off, pulled out a blanket, and splayed it out on the sand. He put the bag on the blanket afterwards, before walking to the edge of the water and slowly wading in, the cold water taking a second for him to get used to as he wadded in deeper and deeper, before not being able to touch the bottom and had to start swimming.

As he swam about, diving under the water and doing small laps around the lake, something took notice of his presence. It was no fish, nor bird, nor reptile or turtle. It was a small larva, as it felt the disturbance of Flail swimming close to where the larva happened to be. Flail kept swimming, hitting the larva which quickly stuck to Flail's coat, sticking onto a single hair follicle and riding down to the skin. It latched on with small barbs, puncturing Flail's skin, feeling like an itch to Flail as he grew tired, having been swimming for a bit and also not being the most physically fit pony in the world. He swam to the shallow, before crawling to the beach, standing up and laying on the blanket.

After basking in the warm glow of the sun for about 10 minutes, Flail pulled out a towel and dried off completely, put his damp blanket and towel into the bag, and few off. The larva this entire time had been burrowing deeper into Flail's skin during this time, already drinking some of the viscous fluids that oozed from the small wounds that the barbs had inflicted on Flail. The blood ran quicker as Flail flew off, returning home from his short vacation, unknowingly carrying a parasite along with him.