In the tree of life, flounders are a sub-sub-…-sub-species of bilaterally symmetrical animals: https://www.onezoom.org/life/@Holozoa=5246131?otthome=%40_ozid%3D1&highlight=path%3A%40Apionichthys_finis%3D3640785&highlight=path%3A%40Bilateria%3D117569#x2913,y-2310,w8.2796

Edit: let me preemptively be a pedant to myself and say that “sub-…-species” is wrong because “bilaterally symmetrical animals” is not a species. Flounder is itself a species AFAIK, not a sub-species of anything. It is a descendant of the common ancestor of all bilaterally symmetrical animals. There, now surely no one will find anything to be pedantic about :D