I think Gorgon had to be the first Chimera that Blackjack fought. He was very dangerous and Blackjack barley won the battle.
But a bit of his back story Spoilers

Gorgon was originally a pegasi soldier called Stonewing and served along side Macintosh and his marauders. He admired Rainbow Dash and couldn’t see that Jetstream was in love with him and during a battle he was shot in the throat and lost his voice permanently. Then later on he was severely wounded and rushed to the ministry of peace medical centre and proclaimed dead. In truth he was sent to project Chimera as a test subject and fused with a Cockatrice. He was put into stasis right after the fusing and was brought out by Sanguine and became a Reaper. He was killed by Blackjack when she got his tail caught into a rock crusher and the rest of him followed.

Fallout Equestria was written by kkat and Project Horizons is written by Somber