I realised that in the last chapter, Dimentio displayed some pony knowledge, despite the fact that in Chapter 2, it was stated that he didn't watch MLP: FiM. Let's just say that Dimentio's a secret brony!
Pony P.O.V.
Humming happily to herself, Pinkie Pie bounded through the streets of Ponyville. She was on her way to the library, where she had heard from somepony that a certain human and pegasus had just landed and were quickly becoming best of friends.
This felt strange to the pink earth pony, seeing as she had seen Rainbow's attitude towards Mr. L, and Mr. L had, presumably, insulted her more than once. That's life, Pinkie! she told herself. Ponies and humans today, tomorrow ponies and griffins!
That's a little meanie-weanie of you, Pinkie, she told herself sternly. Gilda was... okay...
Not exactly, Pinkie argued with herself. She was the meanie-weanie! And she wasn't exactly little, either!
I guess you have a point, Pinkie told her other Pinkie.
Hah! exclaimed the other Pinkie. Victory for Pinkie me!
"WOULD YOU BOTH BE QUIET?" yelled Pinkie angrily. As it so happens, Pinkie had been following Heartstrings and Junebug as she had argued with herself, and the other two had been deep in conversation. Startled, they both turned around.
"Er, were we annoying you, Pinkie?" asked Junebug warily.
"Oh, no!" said Pinkie, returning quite startingly to her normal bubbly, hyperactive self. "Everything's okie-dokie-lokie!"
Once more filled up with the bubbly joy that made her Pinkie Pie, the earth pony bounded towards Twilight's library. She was just about to enter when she heard a discussion going on inside.
" - is dangerous. Don't go near him!"
Was that Mr. L's voice? Pinkie Pie pressed an ear to the door, in an attempt to hear better.
"Well, duh! The guy's creepier than the Everfree Forest and Discord put together!"
Rainbow's voice.
"Well, I was just making sure," snapped Mr. L.
"I'm not a filly, I can take care of myself!" yelled Rainbow angrily.
"I wasn't concerned about you! I was concerned about what trouble we'd be in if Dimentio did something bad! They all think that he's my responsibility." Pinkie noticed the bitterness in Mr. L's voice as he said the last sentence.
"Really? Why?" said Rainbow. Her tone had changed now. It was... curious.
Mr. L sighed audibly, then replied, "We used to be best friends. We'd sneak into Mimi's room to read her diary, get punished by Nastasia for causing trouble, rescued each other from getting chunked by O'Chunks, and played pranks on each other. We used to be inseparable."
How Mr. L said that last word... even Pinkie could sense how depressed he felt!
"But how'd you end up complete rivals?" asked Rainbow.
"That was before he betrayed me," muttered Mr. L.
"Betrayed you?" whispered Rainbow. Of course! She was the Element of Loyalty. Of course she'd be shocked by that.
"That's right," Mr. L said grimly. "And guess how he did it?"
"How?"
"By trying to kill me."