Television pilots are expensive, time-consuming, and can mean a significant investment for studios and networks. Pitches and scripts are tweaked and adjusted, actors are cast and re-cast, sets are built and struck, and a pilot may be filmed only to be heavily re-tooled and re-shot before actually being aired on TV (if it even gets that far). As a result, some studios will film a demo to present to networks instead of shooting a full pilot. The demo is a brief "sample" of what a full pilot, or even full series, could look like. These demos are, essentially, the pilot's pilot.
Demos seem to be under-discussed in both academic and journalistic discourses about television production, but this shouldn't be surprising. Demos largely go unseen by anyone but those involved in the production and the various network executives that decide what gets ordered for the season, and as such, are overshadowed by the full pilot versions. Despite this, demos can be still tell us about the development process of a television program, how things change, and with proper contacts, discussions can be had with creators about that process.
Two examples of pilot demos for you. The first, which you can watch by clicking here, is the complete demo pilot for Buffy: The Vampire Slayer (1997-2003). While it's still very rough, and Willow and Principal Flutie were eventually re-cast, it's very Buffy, full of wit and action, and the demo even shows the "dusting" of a vampire (though it uses very clunky time-elapse animation). Whedon doesn't like to talk about it.
The second, and appearing with this post, is the pilot demo for Batman: The Animated Series (1992-1994, 1997-1998). Bruce Timm and Eric Radomski were both working independently on a Batman cartoon pitch for Warner Bros., and this pilot showcases Timm's character designs and Radomski's "dark deco" aesthetic. The gist of this demo would go onto become the series's opening.
A demo for an animated series makes sense. Production time needs more of a lead between script writing, storyboarding, sending those storyboards overseas to be animated, voice work, editing, and scoring. A fully animated pilot may be even time-consuming than a live-action pilot.
Both examples showcase what their respective pilots would be and what their series would eventually become, and provided executives with the confidence in funding and producing a full pilot and, eventually, series.