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.
Origin of the emote
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.
Spread of the emote
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.