One cool fictional creature that i like is a hydra. A hydra looks like a dragon but has 3 heads and if you cut one off it grows 2 more heads it was a serpentine water monster in greek and Roman mythology. Its lair was the lake of lerna in the argolid the only way you can kill it is if medusa turns it into stone or if you seal the wounds before the hydra grows more heads that's what hercules and iolaus did one of them cut the heads and the other had to seal the wounds with hot iron or a torch so that the hydra wouldn't grow a new head and they buried it under ground. In the movie Percy Jackson The lighting thief they encountered a hydra and the hydra was made by some four janitors that weren't really janitors and they turned into the hydra