Many shogi variants have been developed over the centuries, ranging from some of the largest chess-type games ever played to some of the smallest.
In fact, the largest well-known variant that features the drop rule is the 11×11 game wa shogi.
The elephant was eliminated by the Emperor Go-Nara (reign 1526–1557), and it is assumed that the drop rule was introduced at about the same time, giving rise to shogi as we know it today.
Michael C. Vanier says, "It is thought that the really huge games (dai dai and up) were never really played to any significant extent [...] and were devised merely so that the creators could have the fun of inventing enormous games, amazing their friends and confounding their enemies.
The main reference work in English is the Middle Shogi Manual by George Hodges.
Furthermore, the sources for the rules of almost all the larger variants tend to disagree with each other on many particular issues, even including the very moves of the pieces, such that only for chu shogi and dai shogi is it well-known what the historical rules were, and some small lacunae still arise in the latter with rare situations.
Time will show which if any of the many recently invented variants stand the test of usage and competition from other games, and stay in use.
Ko shogi is unusual for the interdependence of its pieces and the complex rules of promotion, but likewise there is no evidence that it was ever played.