Essentially, the problem is that whenever I have minecraft forge installed, all mobs, including myself, become invisible.
You can interact with them, but its as if the textures are missing. However, if I do play a vanilla version of the game, the mobs are visible.
Through a process of trial and error, I have deduced that this issue stems from the addition of Minecraft Forge versions 18.104.22.168-22.214.171.124.
I am running a single player client on Minecraft 1.2.3
All I have installed at the moment are (in presented order):
MC Patcher 2.3.5_1
Modloader MP 1.2.3 v3
Minecraft Forge 126.96.36.199