So this is just a little oneshot idea I had while taking a shower tonight...Enjoy :)
The first thing he thinks of is the giant water Pokémon Sammy used to be terrified of as a kid. He watches as the monster spits an acidic liquid at an unfortunate werewolf. The werewolf whimpers helplessly as the paralytic venom takes effect. The werewolf's whines turn into panicked howls as the monster towers over him. With slow precision the beast forces its oozing tentacles down its victim's throat until the werewolf is a wiggling mass of limbs disappearing under translucent skin. There is a moment's pause before a bulging red eye swivels towards Dean, and then he's running.
His feet are too loud on the leaf strewn ground. Twigs snap and rip his jeans into bloody gashes as he stumbles over a hidden rock. He pitches forward into a roll just long enough to see the monster trail behind him. It scuttles over the forest floor as if gliding on thousands of little feet. Each step closer is a reverberating beat in Dean's chest. He kicks to his feet and darts to the left; he can see a small crack in a cliff wall in the distance.
Each breath is a burning pain ricocheting in his lungs, but he forces himself forward. In his peripheral vision, Dean can see a vampire shrink into the shadows away from the monster. He wants to laugh because of course vampires are scared of this thing. He just gets all the best monsters, doesn't he?
He squeezes into the entrance of the cave and doesn't stop running until his shoulders are flat against the back wall. Dean searches the cave for any type of weapon and growls in frustration. He doesn't know how long he's been in Purgatory, and so far nothing has helped him find Cas. Hell, he doesn't even know if Cas is still alive. He's truly alone, and now he's trapped, and it's really an awful way to die, swallowed whole by a giant Pokémon, Dean muses.
It is eerily quiet outside the cave. Dean wants to believe the monster has given up, but he's smarter than that. All of a sudden there is a giant roar and mass of tentacles shoot forward into the cave. Dean can't stop the shout that erupts from his lips so he slaps a hand over his mouth. The muddy appendages grope callously at the walls, ever inching closer to Dean. He automatically tries to take a step back and realizes he can't. One slimy tentacle begins to wrap around Dean's ankle and he shoves his other heel as hard as possible against the offending attacker. He presses his arms against the rock behind him and lifts himself up, trying to find purchase for his feet against the smooth wall. He scrabbles upwards, arms trembling with the effort to keep him aloft.
After losing its grasp on Dean, the monster slowly starts to back away, unable to reach farther into the cave. From his vantage point, Dean can see the beast turn, leveling trees and leaving a war path of acid and broken forest underneath it as it scuttles away. Dean slowly lowers himself so his feet are touching the ground. Sweat is burning his eyes but he can still see bloody orbs marking him as a meal.
His knees give and he sinks heavily to the ground. The adrenaline is wearing off and Dean feels a yawning pit opening inside of him. He is alone. His breaths come too fast. He is lost. Spots are dancing before his eyes. He feels tiny and insignificant. He is shaking. He wants Sammy. He can't breathe. He wants Cas. He can't fucking breathe. He wants a long gone Dad and an even longer gone Mom. Help. Cas. Oh god, please just fucking help me.
"Come on, get up," he grunts through heavy gasps. "Get up, get up." Panic is settling in and he wants to cry until his breath hitches and his lungs are burning for oxygen. Suddenly a lone howl echoes outside the cave and Dean pushes himself to his feet.