In the flamethrower's case, it was something that was dropped early in development because it wasn't a suitable alternative to the heavy class' rocket launcher. Meanwhile, the "flying spider shark", called a Seeker in-game, was also abandoned in favor of the Floater, the metallic disc that terrorized soldiers.
Seekers promise to be similar pains in the ass for players, as they cloak and hide their movement til the last possible moment. They will often force players to sacrifice actions to have other soldiers shoot it off a trapped victim's face when it latches on.
XCOM: Enemy Within DLC will arrive on PC and consoles on the 12th November 2013.