Thank you very much for playing, also happy to hear you enjoyed the game! We actually have several art for boss monsters ready. We added the merging mechanic so that two monster can fuse into a bigger and more powerful one, and after 2 merging is done, the image would be replaced with the boss image. But while playtesting we found that usually each monster is merged only once before it is killed or reaching a game over screen, so if we decide to move forward with the game, this will definitely be something we'll work on, among the more different and interesting enemies. :)