Ms. Harshwhinny: [Strutting around the TARDIS, preparing coordinates for their next great adventure] “I just thought we’d try the planet Fellspoon. Just ‘cause. What a good name, ‘Fellspoon’. Apparently it’s got mountains that sway in the breeze. Mountains that move, can you imagine?”
The Doctor: “And how do you know that?”
Ms. Harshwhinny: [Gleefully] “Because it’s in your head. And if it’s in your head, it’s in mine now too!”
The Doctor: “And how does that feel?”
Ms. Harshwhinny: “Brilliant! Fantastic! Molto bene! Great big universe packed into my brain! You know you could fix that Chameleon Circuit if you just tried hot-wiring it into the fragment-links and superseding the binary— binary— binary— binary— binary— binary~ [Halts herself while hyperventilating, holding her head in her hooves for a long moment] “I’m fine! Never been better! You know what, nah, never mind Fellspoon, you know who I’d love to meet? Star Swirl the Bearded! Good ol’ Star Swirl, he’s a legend here in Equestria! Yeah, Star Swirl, Star Swirl- whirl- twirl- pearl- churl- furl~ [Buries her head in her hooves again, the pain in her skull reaching nightmarish levels] “Oh, Celestia!”
The Doctor: [Closing his eyes, taking a step closer to her] “Do you know what’s happening?
Ms. Harshwhinny: [Whimpering] “…Yeah.”
The Doctor: “There’s never been a Pony-Time Lord Metacrisis before now. And you know why.”
Ms. Harshwhinny: [A brief pause] “Because there can’t be.” [Another pause as her eyes begin to water] “I want to stay.”
The Doctor: [Placing a hoof upon her shoulder] “Look at me. Noble, look at me!”
Ms. Harshwhinny: “I was gonna be with you. Forever.”
The Doctor: [Pulling her close] “I know.”
Ms. Harshwhinny: “The rest of my life. Traveling, in the TARDIS. The Doctor-Noble.” [Tries to pull away, fighting to maintain her mind amidst all the emotions and the pain of the knowledge she knows that she shouldn’t possess] “Oh, oh Celestia, no, I can’t go back! Don’t make me go back! Doctor, please! Please don’t make me go back! I can finally do something! I’m not just judging things, judging other ponies, I’m actually doing things! I’m making a difference, for once in my entire life!”
The Doctor: “Noble. My best mate, Ms. Noble Harshwhinny. I am so sorry. But we had the best of times. The best.” [Hugs her one last time before touching his forehead to her own and burying her memories] “Goodbye.”
You know, it’s easy to see Harshwhinny in Donna Noble’s role.
Both of them were harsh and off-putting, but have good side underneath. {The harsh side of Donna seemed to have died away sometime between her first appearance and her inclusion as a true companion.}