I fully think that this should be a tier 5 world unit for either Hippus, Malakim, or Lanun.
HOly CRAPPPPAPPLES!!!
I just had the best Idea of where to place the Hydra.
Its a world unit, has cannibalize, strength 14, 50% vs melee + 50% vs recon + 50% vs horse, gets the montrous unit promotion (so maybe make it stronger in that case) ... and for every unit it fights it gains a temporary unit of strength (5 turns?) which is stackable up to +20 strength (growing extra heads)
perhaps give it whater walking, and require tower of eyes, dungeon, alchemy lab, Strength of Will, and Divine Essence (or Theology if u think DE is a bit much) .... and perhaps 5 incense resources?? or how bout incense, reagents, gems, and gold. (and water mana?)