This is how Thunder Streak's dream and The Tantabus escapes goes in My Little Pony Transformers Prime: Do Princes Dream of Magic Sheep.
[Thunder Streak stands in the old castle throne room]
Thunder Streak: I'm ready.
[A ball of nebula smoke rise in front of the sun and causes an eclipse. Thunder Streak is surrounded by dark magic and he becomes Thunderwing]
[Princess Luna, the Mane 6 and the Autobots show up]
Princess Luna: Thunderwing.
[The Mane Six summon their Rainbow Power and fire their beams at him but he is resistant]
Thunderwing: [laughing evilly]
[Suddenly, the Tantabus becomes aware of their existance]
Thunderwing: No. No!
[It opens a portal to reality and escapes]
[The Mane Six blast Thunderwing again and he turns back into Thunder Streak]
Thunder Streak: What?!
[In reality, he wakes up from his dream]
Thunder Streak: My dream ended happily? That can't happen.