This is how Thunderwing reveals the truth goes in My Little Pony Transformers Prime: Equestria Girls: Legend of Everfree.
[As Danielle follows Danny through the forest, she briefly feels the earth tremble again]
Thunderwing: Nothing. It was me and Sam the whole time.
Sam: Very soon, Camp Everfree will be safe with us.
Thunderwing: Us? Who said anything about us?
Sam: You did. You said we would protect my camp together.
Thunderwing: I lied. As I had from the very beginning.
[Thunderwing grabs Sam's geode necklace from her neck]