Undead – a Necropolis mod.

Is there a mod that replaces the Dark Elf Lizard Riders with Manticores?

A Manticore for level 4? That's quite something. Considering their status, Hydras would be a more fitting replacement for Manticores. But personally, I think the Manticores as they are implemented in Heroes 5 are better off not being seen anywhere (their appearance is unappealing).

Honestly, flying dragons are unnecessary in dungeons. I think it would be better to replace them with Cyclops.

Removing dragons from the Dungeon is a rather unpopular idea.

And give Orcs Trolls instead of Cyclops. Trolls would fit better into the Orc army.

Actually, the Cyclops in Heroes 5 is drawn to look like a Troll, if you compare it to the Trolls from Lord of the Rings. At least, they have a lot in common.

Regarding the knights on dragons, it's clear that they made a mistake with the level (6)?

It seems that Phoenixes and their alternatives were originally planned by the developers for the Elves. But then they were replaced with dragons.

In my opinion, multi-colored dragons are perceived better than multi-colored Phoenixes. On the other hand, in the original game, all High Elf dragons are green clones. But a version with an emerald/gold/azure dragon would look better.

As for Phoenix alternatives, it's unlikely that the developers planned them, since at that time there were no alternatives at all. As far as I remember, the developers also didn't provide models for firebirds, which suggests that Phoenixes were never planned as a castle unit. But nevertheless, based on previous games in the series, Phoenixes instead of dragons are a viable option.

As for the new Orcish units, unfortunately, the model is ugly.
Ogre mages - it would be better to have ogres as mages. And in general, the parallel between overgrown Orcs and Ogres is really interesting.