Peter Koenig was a private in General George Washington's Revolutionary Army who arrived in the Land of the Lost some time prior to the Marshalls. Koenig and his companion Harry Potts explored the Land extensively in search of a way back to Earth.
When the Marshalls first arrive, they discover a stone pillar in the jungle where Koenig had written "Beware of Sleestak" with chalk. The Marshalls only learn much later who had been responsible for this warning, when they discovered Koenig's journal. Koenig claims to have dubbed the Sleestak thus in honor of Major Joshua Sleestak. Koenig claims in his journal to have taught the Pakuni a few English words.
Koenig met his death when he followed Potts' path into the The Lost City. Koenig went into the third entrance, "where the pillars end," which eventually led through a narrow passageway into the "Devil's Cauldron". This was a lava chamber, with a narrow ledge, where he became trapped when the lava level rose, reviving the Sleestak. He preferred to await death in the lava chamber, rather than facing the Sleestak who had gathered outside the chamber to catch him. The Marshalls find Koenig's skeleton in the chamber and narrowly escape a similar fate.