When the Megaspells detonated across Equestria, many earth ponies, unicorns and pegasi took refuse in Stables to avoid the devastation. Special Stables were made to house griffons and hippogriffs of all sorts, but the merponies weren't privy to the safety of the Stables and were left to the dangers of the radiation that seeped into the oceans and waters everywhere, twisting them and corrupting them- turning them into monsters and whittling away at their minds until they no longer remembered who they were.
The merponies have been mutated into something like Ghouls, their teeth turned into fangs, their fins getting sharper, their hooves cleft. They began to feed on meat and eachother, uncaring of the lives they took and only focused on violence- whoever they were before has been lost over the ages. The Oceans aren't safe, the beaches are prowled by these creatures that hide in the shallows and spring out at unsuspecting victims.
WaveRunner was one of those few who made it to a Stable, but was unaware that the Stable-Tech workers intended to force it's tenants into cryosleep. She woke up unawares of what had happened to the world, and the Stable around her was dead and silent. The Wasteland was a shock, but more so was the state the merpony's and wildlife had been thrown into.
Faint of heart and jumpy soul WaveRunner found herself entirely unprepared to the sight of the merpony that had once upon a time had been her friend- Sea Bastion ("Sebastion" as she called him).
And he was snapping at her heels, hissing and snarling with an animal hunger in eyes she didn't recognize- she hardly recognized him, beside the color pallet!
A little Fallout 4/MLP AU version of
MLP to Hasbro
OC's and Designs, Story to Me
Fallout 4 to Bethesda
FoE to Kkat