I am no expert on this section. Basically there are mammoth modders resources available so we need someone to take the following and add them as a plugin for the Mesogea mod. You will need to have the Mesogea main master file. Simply make the plugin, get the beasts working realistically and added into the construction set:
Mountable mammoths
Wild mammoth male and female, and children if possible.
The resources is here:
http://www.tesnexus.com/downloads/search.phpmammoth mounts