Spike has a secret.
One even Twilight doesn't know.
He has a lust for pony urine...

When he and Twilight go on a picnic with their friends, and Rarity suddenly has to pee, Spike finds the ensuing events relevant to his interests...

In case it wasn't obvious, this contains heavy watersports/urolagnia material.
If you're not into that sort of thing, don't be a dick and downvote due to the content - just go read something else.