[Malsomnus]

First thing that pops up for me is the line:

local f, g, a = 1

Is it possible that what you meant to write is

local f, g, a = 1, 1, 1

?

[Edit]
It really looks like lines 18, 20, 22 and 24 should complain about indexing/ arithmetic with nil values. Do you get these errors?
Anyway instead of managing indexes you can just use:

table.insert (mounts.flying, {creatureID, creatureName, creatureSpellID, icon, issummoned, mountType})