Binky Pie

Pinkie Pie is minding Sugar Cube Corner one slow day when an... unusual customer answering to the name Bill Door arrives. It seems Mr. Door was an old friend of Pinkie's grandfather, and he has an unusual job offer for Pinkie Pie...

Yes, Pinkie Pie becomes the new steed of Discworld's Death, on a part-time basis at least. But after a bit of a mix-up with the Rite of Ashk'Ente being cast on two planes at once, Pinkie Pie is stuck on the Discworld doing The Duty, and Death is stuck in Equestria in his guise of Bill Door. On top of all that, it seems Pinkie's antics have inspired a zombie to start a new religion...

Can be found here: http://www.fimfiction.net/story/400/Binky-Pie

A God Am I: Pinkie manifesting in front of a temple full of her worshippers leads to official apotheosis. Unfortunately, it also leads to Goddess!Pinkie and Death!Pinkie becoming two seperate entities.
Black Cloak: Pinkie reluctantly dons one when she takes on Death's duties after being told they don't come in pink.
- Rarity also displays some fascination in Death's own due to its unique fabric.
Deaths Hourglass: A recurring motif between chapters. When Pinkie takes on The Duty, her own takes the form of a slowly-deflating balloon in Gothic Pink.
Flat What: Gustav Gutsworth's reaction to being greeted on "the other side" by Pinkie Pie.
God Guise: Pinkie Pie accidentally inspires people to worship her as "The Pink Pony of Death".
Names the Same: Mrs. Cake of Ponyville and Mrs. Cake of Ankh-Morpork. In one of the multiverse's amusing little coincidences, Equestria's Mrs. Cake has precognitive powers like Discworld's Mrs. Cake.
Not What It Looks Like: Between "Bill Door" and Fluttershy.
Oh Bugger:
- Death's reaction to finding he's been transported to Equestria, and Albert's reaction to Pinkie Pie ending up in his place, in chapter 3.
- The Auditors in chapter 9, on realizing Pinkie's ascension to goddess-hood places her outside their jurisdiction.
Posthumous Character: Pinkie's grandfather, Binky, appears in a lengthy flashback in the prologue, but is dead by the time the story begins.
Plot Triggering Death: The death of Binky Pie, Pinkie's grandfather and Death's steed, is what begins the story.
Shout Out: A few nods to The Hitchhikers Guide to The Galaxy; it's noted in chapter 9 that every society in the multiverse has developed a drink with a name similar to "gin and tonic", and in chapter 10 there's mention of a man making a mountain invisible with a fleet of intergalactic trucks.
Tome of Eldritch Lore: The Neighcronomicon, also known as the Necroponicon, Equestria's answer to Discworld's Necrotelecomnicon.