Flower Dragons were created by RKS though were made popular by Cibryll due to their ties to Bloom Seymour, a sockpuppet who became popular due to her bizarre yet endearing personality and her tendency to reference flower dragons with the :3 emoticon at the end.
Tiamat's sprites were reused from Curse of Il'andriel, one of Shane (aka Cibryll)'s unfinished mods. In that mod, they aren't fought and their sprites go unused because the last boss is Cave Story's final boss.