Since they’re so popular now, I might as well reveal the others’ Metaphorically Visible Emotions, or MVEs.

• Apple Bloom: Natura the Apple Blossom of Maturity. It’s actually Apple Bloom’s thoughts on maturing, so it frequently yelps out, “You are now in heat” or, “ There’s Scootaloo, stare at her haunch.” Like Scootaloo, Bloom has the problem where she sometimes takes Natura seriously. >_>
• Dinky: Ben the Empty Black Dwarf (Star). Basically, it’s the same thoughts that Sweetie had succumbed to; nihilism and the emptiness of reality. Unlike Sweetie, Dinky chooses to ignore it almost completely, which is why she's intelligent, yet spontaneous and positive.
• Rumble: Clara the Panphobic Cloud. It’s all possible fears wrapped up in a wispy pack of bullshit. He chooses not to listen to it… unless it’s something that he's legitimately afraid of. Examples include large bodies of water, wildfires, and sometimes even Scootaloo.

Don’t get too attached to them, though. Only Clyde plays a key role in PC. The others are just there to acknowledge that they exist (aka your entertainment). You might not even see them all. :/