I've been playing the game (Genesis) for quite a while and have a question regarding army recruitment. I've found that you can get almost all units in your army, but there were a few that I haven't managed to get yet and was wondering if it's possible to recruit them.

Mainly they're the giant type units that you find in ogre or giant lands. I haven't found:

ogres
unicorns
cyclops
giants (although I had them as part of a guard contract)
dragons (probably can't recruit unless you count dragon transformation)

I also have only found medusas as mercenaries and was wondering if they can be recruited in any other way.

For unicorns I think there's an exploration event that allows you to acquire a unicorn if your hero has the right skills.

Hmm, ok. I was hoping they would be available as something other than a mercenary since it's always a pain having a medal slot taken up and they're worth twice what they should be.

This is easily moddable, I know about mods with medusa and dragon eggs.
For other creatures it is possible to include "contracts", which would be eggs by mechanic.

Like New Horizaons mod :)

I have no idea how much trouble this would be to mod in, but I could also see the possibility of a "Fantastic Summons" ritual that recruits a creature of this (or a similar) sort to fill an available army slot. This would probably involve paying them mostly in crystals for thematic reasons, but it might be more accessible than the random drop approach.

Dragon's egg mod.
http://www.gog.com/forum/eador_series/moddingindexes/post5