In the Pony episode Too Many Pinkie Pies, Pinkie creates several clones of herself using a magical pool she finds in a cave.
The clones apparently have none of her memories, seeing as they fail to recognize the faces and names of those whom Pinkie is deep friends with.
The clones wreak havoc throughout Ponyville and are indiscernible from the original Pinkie.
Eventually, Twilight Sparkle rounds up all the clones, including Pinkie herself (without knowing which one she is), and puts them to the test: stare at a wall of drying paint. First one to lose attention gets vaporized. The real Pinkie, having her own real memories and feelings for her friends, would overcome her attention difficulties and stare at the wall the whole test long, and would be allowed to survive. The test apparently works.
One problem: if the clones are born with no memory, how are they able to speak?
If they can remember how to speak, the only reason they don't recognize Pinkie's friends is because they don't care about them at all. Their one desire is to escape in the doppelganger bodies they've just acquired and do as they wish at the expense of everyone else.
They don't want the trouble of having to help Pinkie and her friends...so they lie. They lie to Pinkie, saying they don't remember Pinkie's friends.
Because they have no empathy.
That is the reason for their markedly chaotic behavior. If they can remember how to speak, they should remember Pinkie's friends. The difference: they just don't care.