MLP/Undertale crossover.
Not a PWNY-verse story.

Long ago, the Monsters and Humans fought a great war. The Monsters lost, and were sealed in the Underground. It would take the power of seven human souls to break the barrier that kept them imprisoned. They only had six. And so the Monsters waited and hoped, praying that another human would find their way to the Underground. Some thought the seventh soul would give their King the strength to break the barrier. Some dreamed of a human who would be their friend, and save them all...or had nightmares of one who would bring back the war.

...but nobody came.

...but somepony did.

Eons after the war, with no way to tell how much time had passed above, a tiny blue filly with wings and horn fell into the underground, calling for her Mama. None knew the poor creature's nature or origins...but her soul was powerful with magic, even more so than a human soul.

None can know what will come...of this little 'Woona'.

Cover art by Sanyo21

In a remote corner of the Solana Galaxy lies a backwater planet called Veldin. Nothing important ever happens there. Nothing exciting, either. Ratchet, a Lombax with no idea of his origins, is beginning to grow bored with his life, and dreams of excitement. He wishes on a falling star for some excitement to find its way into his life.

The falling star answers by crashing into his home in the form of a basket carrying a tiny creature with a horn, a pair of wings, and four hooves. The name on the basket reads "Twilight Sparkle".
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Part of the PWNY-verse.

Edit: Now with cover art by the amazing G_Haze
Video Consultant/Technician: m2pt5 (he provides the infobot videos and similar.)

Edit: Game Timeline
Ratchet and Clank: Done
Going Commando: Done
Up Your Arsenal: Done
Deadlocked: Done.
Size Matters: Done.
Tools of Destruction: Done.
Quest for Booty: Done.
Crack in Time: Done.
All 4 One: Done.
Into the Nexus: Done.

Jak never found it unusual that he had a unicorn brother. Shining Armor had been there his entire life, as far as he could remember. Sure, Daxter often made a point of just how strange it was relative to everyone else in the village, but then again Jak himself was different from the rest of the villagers.

He never questioned that he looked somewhat different from everyone else. Never questioned that he was raised by Samos the Sage, alongside Samos' daughter Keira, whom he kinda-sorta liked and who kinda-sorta liked him, even if Samos himself didn't like that. Jak, by nature, wasn't the sort to question things.

Shining Armor, on the other hand - or hoof - had lots of questions, but no answers.

...yet.

Part of the PWNY-verse.

Crossover idea suggested by Keyblade Hero.
Cover art by Sanyo21