Let's take a look at the 12 (the order is not
chronological):
1) Nemea Lion
2) Calydon Boar
3) Hydra - Water Serpent
A three-headed version can be the original? The triplet
foe? A Vrtra-like monster?
4) Cerynia Deer
5) Cretan Bull
The association with Creta must be a later change. From
Creta = From South? From Sea? A kind of bovine sea monster? Relation with
Theseus and Minotaur) The Cosmogonic IE Bovine?
Gilgamesh x Heavenly Bull ?
6) Diomedes Mares
Some kind of female demons?
7) Stymphalis Birds
Demonic Birds? Like Cuchulain capturing
swans?
8) Amazons
I think Amazons were originally a kind of sea-women (like
Sirens)
9) Augias
Kerberos - the Underworld Dog
10) Hesperis apples
Fighting against Ladon, other serpent.
Gilgamesh travel to Western realms?
11) Geryon
The typical IE Triplet Foe. Fighting against Orthros,
another dog.
Important: Everyone is looking for a pattern... but is it necessary
a pattern? And if Herakles's labors are just a collection of
independent myths?