Which units would you replace in the Dark Elf faction?

Here's an idea about Hydras: make them either a Tier 7 unit or a neutral creature, but the best option would be to place them in a swamp and make them a Tier 7 unit there. The idea is this: it becomes incredibly powerful, and it has its mythological ability: it initially has 9 heads. Every time it takes any damage (even 1 point), it gains +1 head. The more heads, the more damage it deals. Damage: all heads' damage is summed up and divided among all nearby enemies. Each of them receives damage equal to the resulting number. However, it is vulnerable to fire. Fire deals more damage to it, and instead of gaining a head, it loses one (-1 head, and consequently, the damage decreases). The Hydra dies when it runs out of health (and it has a lot of it) or when it has no heads left (they are removed by fire). In a fully upgraded city, it would spawn one every one or two weeks (with the Grail, two). And another feature: a hero can only carry one such creature in their army. They cannot be stacked. Or, you could allow hiring one per hero (then without any growth) or something similar. That's it. It could be implemented as a mod for SnowApe (the Yeti).