Late one night, a small group of celestial beings and deity's were playing with the fabric of space-time, toying and twisting it with their immense powers, creating something that no-pony had ever seen before.
They were drunk.
Very drunk.
And so, the morning after the night before, it came to be that the Legend of the Toilet Door was created...
And not a single crossover happened.
(Note: Happens somewhere between Season 2-3.)