This is how Princess Twilight becomes a Dragon goes in Re-telling The Legend of Everfree.
[Optimus suddenly notices a bunch of Dark Energon, which Twilight had just found]
Optimus Prime: What the...?
Bumblebee: Dark Energon?! But, that could mean- [gasps] Unicron! Twilight, get away from that before-
[Twilight absorbs all the Dark Energon and starts to undergo a transformation]
Knockout: Too late.
[Twilight's wings turn into dragon wings, her front hoofs turn into dragon arms, her back hoofs turn into dragon legs, her skin turns dark purple and black, grows a long neck, face turns into a dragon face, and her eyes turn red. Dragon Twilight roars.]
Dragon Twilight: [roar]
Rainbow Dash: Twilight!
Pinkie Pie: What happened?
Rarity: Seriously?
Applejack: No!
Fluttershy: Oh, dear.
Starlight Glimmer: Oh, no.
Trixie: Oh my.
Thorax: What?
Discord: How?
Princess Celestia: Princess Twilight!
Princess Luna: What happened?
Princess Cadance: Twilight.
Optimus Prime: What?
Bumblebee: How?
Arcee: Come on!
Bulkhead: Bring it on!
Ratchet: Yeah!
Smokescreen: Alright!
Wheeljack: Time for some action!
Ultra Magnus: Lock and load!
Knock Out: Yeah!
Dragon Twilight: I'm not about to let you stop Unicron.
[Dragon Twilight grabs Princess Celestia, Princess Luna, and Princess Cadance]
Dragon Twilight: Not this time!
Princess Celestia: Twilight!
Princess Luna: Think about what you're doing.
Princess Cadance: Please.
Dragon Twilight: Sorry. But Princess Twilight is gone. And once I find more Dark Energon, I'll be the most powerful being there is. So how does it feel knowing that every Pegasus, Unicorn, and Earth Pony will bow to Lord Unicron's will and that there is nothing you can do about it?
Princess Celestia: Twilight.
Princess Luna: Please.
Princess Cadance: Stop this.
Dragon Twilight: Ha! Nice try, princesses.
[She flies away with them]
Optimus Prime: Oh, no! The princesses!
Bumbleebee: [falls to his knees and bangs the ground in frustration] Unicron, you maniac, you corrupted her! Darn you! Darn you all to heck!
[Thunderwing, who was hiding in a bush, watches with glee]
Thunderwing: Oh goody, Unicron is really gonna love this, with Twilight on his side, this planet will fall faster than I anticipated. [laughs before flying away]
[At Dragon Twilight's castle, Dragon Twilight places Princess Celestia, Princess Luna, and Princess Cadance in a cage and swallows the key to unlock it.]
Princess Celestia: Twilight.
Princess Luna: Please.
Princess Cadance: Don't be like this.
Dragon Twilight: It just had to be this way.
Princess Celestia: Why?
Princess Luna: Why are you doing this?
Princess Cadance: Who are you working for?
Dragon Twilight: For Lord Unicron. Now if you excuse me, I have a call to make.
[Dragon Twilight activates a device and places it on the floor and Unicron's face appears in a hologram and Dragon Twilight bows before him.]
Dragon Twilight: Lord Unicron, I live to serve you.
Unicron: Yes.
Dragon Twilight: I gathered the Dark Energon you wanted me to gathered.
Unicron: Good.
Dragon Twilight: We need to get a few more doses and then you will be the most powerful Transformer who has ever lived, my lord.
Unicron: Yes.
Dragon Twilight: Come, let us talk.
Unicron: Of course.
Dragon Twilight: I'll be having offspring in no time.
Unicron: With your offspring, no one will stand in our way.
Dragon Twilight: I know.
Unicron: So, how's the Dark Spark?
Dragon Twilight: Doing great.
Unicron:
Dragon Twilight:
Unicron:
Dragon Twilight:
Unicron:
Dragon Twilight:
Unicron:
Dragon Twilight:
Unicron:
Dragon Twilight:
Unicron:
Dragon Twilight:
Unicron:
Princess Celestia:
Princess Luna:
Princess Cadance:
Dragon Twilight: No you don't understand, Celestia, Luna, and Cadance. Dark Energon makes me stronger and powerful. And I love it.
Princess Celestia:
Princess Luna:
Princess Cadance:
Dragon Twilight:
[Dragon Twilight leaves]
Princess Celestia:
Princess Luna: Well she's now ferocious, she's amphibious, and... she's pregnant.
Princess Cadance:
Princess Luna:
Princess Celestia: Wait how can she be pregnant? Doesn't she need a mate?
Princess Luna: Not if she reproduces asexually.