In case there can't be any Transformers in the My Little Pony: Friendship is Magic Universe, here are my ideas for two existing Decepticons

Starscream(Voice: ether Michael Dobson or Steve Blum)

Slipstream(Voice: Tara Strong)

Starscream and Slipstream are Hippogryphs, formerly from Mt. Aris and Seaquestria.  Once they served Queen Novo.  But when the Storm King invaded and sent the Hippogryphs to Seaquestria, Starscream and Slipstream saw a glimmer of high power within the Storm King, as they were disappointed in Queen Novo's desire to become strong.  WIthout a second thought, they took sides with the Storm King, to the shock of their brothers and sisters.

But what shocked Queen Novo and the Hippogryphs, is that Starscream and Slipstream tried to betray the Storm King for their own gain.  The duo were ousted from the army of the Storm King and were left wandering the planet, desiring power for themselves.  As of that event, STarscream and Slipstream are no longer welcomed on Mt. Aris or Seaquestria, not just because they took sides with the Storm King, but their willingness to betray him just for power.