How do you create monsters by combining multiple creatures?

A random composite unit can be assembled using scripts, but this is not a trivial task. In the RM mod, there is a script that replaces ordinary random units with composite ones. You can take it from there.

Added 1 minute ago

Medyan, did you manage to do this in Heroes of Might and Magic V? As far as I know, there is only a script for creating a single monster, and you can only add to neutral units those creatures that they already have, so you cannot assemble a composite unit using scripts.

In Heroes of Might and Magic, it is possible to create combined armies using scripts.