What's interesting about guest characters in Mortal Kombat, is that even though they've become rather synonymous with each other, it wasn't until the most recent MK game that they became a robust feature.
See, the first Mortal Kombat to feature guest characters was Kratos in Mortal Kombat (2011) - although he was a PlayStation exclusive. Prior to that, we got Mortal Kombat Vs. DC Universe, which didn't technically feature any guest characters, but was chock full of DC superheroes, from Batman to Captain Marvel. As that was its own release, don't expect any of those to pop up here.
All of which is a long-winded way of saying, guest characters in Mortal Kombat haven't been around as long as you may imagine, and there probably haven't been as many as you think. While the list may be small, it's chock-full of awesome characters that fleshed out the roster, while ripping the flesh off other combatants.
As we gear up for another year of Mortal Kombat goodness, with the impending release of MK11, it's time to reflect on the horrors that have come before.