BtVS by Whedon and Mutant Enemy. "Fish" by Jen LeMaire. The Land of Xanth created and populated by Piers Anthony; most of this happens inside the Gourd which is a lot more surreal than mainland Xanth, but also has fewer puns.
This is set immediately before 'Night Mare'. Some spoilers, but not many.
Jinni, one of the Twisting the Hellmouth admins, posted a Challenge asking for crossover stories inspired by Jen LeMaire's 1990 eight-line poem "Fish". Sounds fun. I guess I'll take the lines, found at sithspit dot com / jen / fish dot htm , one at a time.
While on patrol, Buffy heard some cries of pain and came running.
Gee, this really brings back memories of old movies she thought as she arrived on scene. Funny, those don't look like cowboys... Looks like I'm going to be breaking a few necks tonight.
There were a half-a-dozen short green demons pestering some horses. She'd killed some before, but she'd never bothered to ask Giles what they ate.
From the sharp stick with a spigot on the end they were trying to shove into the animal's flank... Ewww...
"This is a one-horse town," she drawled loudly.
They spun around to face her.
"It's simply not big enough for-" Wait a second. "Horses?"
The two animals were frightened, their eyes rolling back and forth.
They were tied up to the bike rack in front of a bookstore...
They looked nearly as out of place in Sunnydale as that glowing portal she saw out of the corner of her eye.
Nope. They looked more out of place.
Weird, she thought as she raised her stake.
The demons had gotten over their shock and had, snarling, begun to advance.
And from the sound of it... Yep, she was surrounded.
She spun around suddenly and staked the first one that had tried jumping off a roof and onto her.
There were more where that one came from.
It was messy work. She was way outnumbered.
She could've easily left, but...
Fortunately, she didn't have to.
The sound of cracking bones alerted her to a friend in the fray.
She spun around and took in the sight of the massive white stallion squishing demon skulls left and right.
When the last of the smelly things were dispatched, the gorgeous horse reared back on his hind legs, let out a triumphant whinny, and ran off into the night.
She stared after the beast, then walked up and tried to calm the two still tied up.
After a bit, they let themselves be petted.
"Hey, I'm sure your owner will be coming back through here... That portal looks like his way home... I'll just go and kick his butt for leaving you alone on a Hellmouth. Alright?"
One of them nickered pleasantly.
From the top of a roof she saw a strangely dressed man in the middle of a circle of vampires.
"Look. We don't need a refugee from a fairy land coming here and askin' favors of us." A vampire snarled as he shifted into game face. "We're not going to leave this place of buildings and easy prey for an open forest, no matter how tempting the pay is."
"Matter of fact, Horseman, " the vamp practically yelled as he walked closer to the stranger. "Me and my gang here are just going to take this insult outta your hide."
The smirk was wiped off his demony face when a bolt suddenly thudded into his chest.
He was reaching to pull it out, when he dissolved into dust.
Buffy made short work of the other vamps, then trained her crossbow on the man.
He was dressed in brown, in a practical style centuries out of fashion. He had a scabbard fastened about his waist and a brass ring around a wrist.
She'd seen something shiny on the white stallion as well. Probably two of a kind.
He was handsome enough, but something about the smile he was giving her made her want to stay on the roof.
"What? You want me to call the ASPCA? You can't just let animals run loose on a Hellmouth!"
"You must be a hero," the guy said, his grin growing wider.
"Pretty much, but it's not like I asked to be one."
The man raised his left hand and...
Buffy didn't move, but she could suddenly see behind the man, as if she was a movie camera being pushed along a track.
She suddenly could see the portal, way across Sunnydale, and the lush green of the forest through it.
She was forced to watch the edges of the portal disappear behind her, the trees fly by, as her point of view neared an odd patch of squash.
They all seemed to have holes in them and, as her line of sight drew nearer, she-
The Horseman grinned so much the top of his head nearly toppled off, but he was able to hold it in place.
The blonde girl slowly sank to her knees and sprawled limply across the roof.
The young woman, whoever she was, had held on longer then most.
In the half-second of time available she'd even managed to close her eyes.
Of course, it hadn't helped.
One less worry, he thought, then frowned. But also one more kind of 'mundane' monster that I can't recruit... On to the next.
The previous Waves of Conquest trying to take over Xanth had mainly failed because they all drew from mainland Mundania, with its purely human stock.
He had finagled his way here, to an alternate Mundania, in search of the monsters and the foreign magics that filled this specific reality.
So far, he hadn't had that much luck...
Buffy blinked, then blinked again.
I hate portals, she thought.
"At least this one's interesting."
She looked around her.
The sky above was as white as an overturned bowl and the ground, no... The liquid below looked like a sea of milk with cheerios floating in it.
She was on a small plank of wood floating in mid-air.
There, too far for even her to jump, was another board floating at a lower height.
Just above the frothy surface of the lake was yet another board.
Yep. Nothing in reach.
As a next to last resort she yelled for help.
On receiving no answer... She took a deep breath, rolled her eyes, and dove from the highboard.