Hydras in my world are much like a brontosaurus in form, but has three heads. Much like a hippo, hydras have incredibly dense bones, and sink rapidly in water. Hydras are almost exclusively swamp creatures, using their long necks (Which are like a large snake in shape) to keep above the water. Like a hippo, Hydras have hooves, (Despite being a reptile.) and have the mobility of a hippo underwater. Hydras have limited magic- the most ancient hydras can alter their own density slightly, and every mature hydra has access to a boiling acidic venom (It still must get into the bloodstream to have any effect.) they can spit at reasonable distances. (Non-magical spitting.) Hydras cannot fly. The most important aspect of the Hydra, is it's incredible regeneration. If a Hydra's head is cut off, it will grow 2 in the place of it, reasonably rapidly, (This is magic.) The energy does come from the hydra, so doing this enough will exhaust it, but it can use it's stores of fat and dense bone marrow to fuel this. My main concerns for this creature, is that it will just sink into the swamp, or be incredibly inefficient. Can the swamp floor support such a massive creature? Hydras can weigh over 50 tons, and if they grow too many heads without shedding, they can die. If a hydra would sink, is there any way to prevent this? Would adding two more legs still allow it to run as fast, or even solve the problem, if any? If it is impossible to not sink, how can a hydra escape the bog?
Other questions about hydras do not solve this, and I can't find anything on how things sink into the swamp, but I do know they do. Pygmy dinosaurs have supposedly been spotted in the Congo, so at least we know that type of creature can live in the swamp. (Maybe?)