Early Works

The early works on Invader Zim: The Conquering began around 2005, when Jhonen made a few storyboards of Invader Zim, the first one was the Pilot, which was released in 2008. Jhonen made a few ideas on further episodes, nothing was said about a new IZ show until 2012, when Jhonen announced that he is making a brand new show with Invader Zim and ponies. A short scene from The Lie was revealed in June 15, 2012. In December 20, 2012, another scene was revealed. Finally, in February 2013, The Lie aired.

The Idea Of Ponies
The idea of adding ponies in the show remains unknown, however they appeared in the first season of the show, they were meant to be added in the pilot, but this was later rejected. Other information about ponies being in the show before isn't known or hasn't been seen.