MetaMount scans your inventory and chooses a random mount from the mounts available to you when you activate it. It distinguishes between zones which are mount-restricted or mounts which are zone-restricted; i.e. in Ahn'Qiraj it will pick a random bug mount, while not trying to select bug mounts when you are not in Ahn'Qiraj.
Commands:
/mm or /metamount
scan - scans the mounts available to you
list - list the mounts known to metamount
mount - mount!
help - list the help
To use:
make a macro with '/mm mount' in it, or use the keybinding provided by the addon.

