If you allow weapon type adjustments in your campaign please be certain to remember that these adjustments are for weapons versus specific types of armor, not necessarily against actual AC.
In most cases,
monsters not wearing armor will not have any weapon type adjustment allowed,
as monster armor class in such cases pertains to the size, shape, agility, speed, and/or magical nature of the creature.
Not excluded from this, for example, would be an iron golem.
However, monsters with horny or bony armor might be classed as plate mail if you so decide,
but do so on a case-by-case basis.
Naturally, monsters wearing armor will be subject to weapon type "to hit" adjustment.

<Second draft>

<similar to the above, but just list monsters with +10 or greater natural armor in 3e = AC 0>
<as well, they must have an AC of 0 or lower in 1e>

<
astral deva
planetar
solar
trumpet archon

behir
beholder
bulette

balor
bebilith
glabrezu
hezrou
marilith
nalfeshnee
retriever
vrock
barbed devil (hamatula)
bone devil (osyluth)
horned devil (cornugon)
ice devil (gelugon)
pit fiend
triceratops
black dragon, juvenile +
blue dragon, young +
green dragon, young +
red dragon, young +
white dragon, juvenile +
brass dragon, juvenile +
bronze dragon, young +
copper dragon, young +
gold dragon, very young +
silver dragon, young +
dragon turtle

earth elemental, 8 HD
earth elemental, 16 HD

formian myrmarch
formian queen

stone giant
storm giant

clay golem
flesh golem
iron golem
stone golem

gorgon

annis
green eyes

9-headed hydra
10-headed hydra
11-headed hydra
12-headed hydra

<begin at I, page 158>
 

etc.
>
 
 

<First Draft>

<NAC=Natural Armor Class>

<+9 natural armor in 3e = AC 1>
<+10 or greater natural armor in 3e = AC 0>

<
NAC
achaierai = 1
astral deva = 0
planetar = 0
solar = 0
ankheg = 1 <>
hound archon = 1 <>
trumpet archon = 0 <>
barghest (greater) = 1
greater basilisk = 1
behir = 0
beholder = 0
bulette = 0
chimera = 1
couatl = 1

etc.
>