From NWN1 Custom Content Guide

[edit] Overview

1.69 adds horses that DLA developed for BioWare for the "WyvernCrown of Cormyr" premium module.

[edit] The Horse System

This is a fairly complicated system but once you get how things fit together, it's actually very easy to work with (and expand and add more custom mounts!)

Every part-based player model references a relevant P-model of the format of p(gender)(race)(phenotype) like pmh0.mdl for 'Regular Male Human' or pfe2.mdl for 'Fat Female Elf'.

The same goes for the robes and cloaks, which are applied to the character by overlaying the model on top (hence the occassional animation misalignment).