Zappa is the Halloween version of the original Twitch.tv emote Kappa. Obtainable when using the BTTV (Better Twitch TV) or FrankFaceZ browser extension on site.
The two variant different look to both of them: the BTTV variant is a zombified version of the original Kappa emote while the FrankFaceZ is a very dark Kappa face, signifying that it is hiding in the dark.
Both meaning are still the same as the original Kappa emote: they are both used when trolling the streamer or to let others know that your are lurking in the chat.
Based on a picture taken of Justin.TV employee (now known as Twitch.tv) Josh DeSeno, it was one of the original Twitch.tv emotes, uploaded in late 2015, 4 years after its original version Kappa has made it to the site and dominated the chat rooms.
Zappa was added to the BTTV (Better Twitch TV) collection a few weeks before Halloween in 2015, according to a Twitter post by the BTTV staff.
As of 10/12/2019 according to the website StreamElements, Zappa is the 82nd on the list of most popular/used emotes on the website Twitch.tv, making it one of the more popular emotes.