Fennekin

Fennekin was one of Serena's Pokemon, along with Pancham.

History
Fennekin made her first appearance in "Ash Ketchup Vs. Serena" as a Braixen where she battled Ash and Pikachu

She re-appeared again in "Serena cuts her hair" where her and Pancham try to cheer their trainer, Serena up. They both watch as Serena shaves her head.

In "Why Serena isn't in Alola", Serena prepares to head off to the Alola region, more than likely with Fennekin (Braixen) and Pancham. This would mean that along with Serena, Fennekin was killed in the Team Rocket terror attack on the plane.

Shorts
Serena cuts her hair

Other
Ash Ketchup Vs. Serena

Trivia

 * Though it has not been officially announced, it can still be assumed that Fennekin was killed in the plane explosion.