It is possible for polyploid organisms to revert to lower ploidy by haploidisation.
Dihaploid and polyhaploid cells are formed by haploidisation of polyploids, i.e., by halving the chromosome constitution.
Dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from the diploids, for example by somatic fusion.
Some eukaryotic genome-scale or genome size databases and other sources which may list the ploidy levels of many organisms: