In principle, one could construct a non-elementary knightship (the Geminoid construction is said to be capable of this - essentially it's a giant universal constructor that can be "programmed" to reconstruct itself an arbitrary distance from its initial position), but they're rather less interesting since they tend to be very large and not useful as building blocks.

In contrast an elementary object, like a knightship, tends to be nice and small, and therefore highly valued as a building block for more powerful systems. Orthogonal and diagonal gliders see a ton of use in complex Life systems, and it's probable that the new knightship will see good use too (once people start building and exploring the requisite tooling - guns, eaters, and other reactions).

See https://niginsblog.wordpress.com/2016/03/07/new-spaceship-sp... for a rundown of the difference.

It is definitely possible, and extremely probable, that extremely large "elementary" structures exist - but we lack the computational power to find such things.