This is what I came to these forums to find out about, back at the beginning of the year. I wanted a Wardog unit to add as a mercenary unit out on the steppes. When I asked at the time, the response I got was a simple 'no'. Nine months and my own mod later, and with just a few hours of trial and error and I know there is a way after all.

Step 1: EDU
At the top of EDU, commented out is the files info section. Here there is a reference to animal.
Spoiler Alert, click show to read: 

; soldier Name of the soldier model to use (from descr_models_battle.txt)
; followed by the number of ordinary soldiers in the unit
; followed by the number of extras (pigs dogs, elephants, chariots artillery pieces etc attached to the unit)
; followed by the collision mass of the men. 1.0 is normal. [Only applies to infantry]
; officer Name of officer model. There may be up to 0-3 officer lines per unit
; ship Type of ship used if applicable
; engine Type of siege engine used by unit
; animal The type of (non ridden) animals used by the unit
; mount Type of animal or vehicle ridden on

So we create a new unit with the 'category' set as handler, (I used the existing AS Ballista, as it is obsolete in my mod). Its easy enough to clone an existing unit. descr_formations refers to handler, so we know it can be used.
Spoiler Alert, click show to read: 

begin_block 6
unit_type handler 1.0
unit_type siege 1.0
block_formation line
block_relative_pos 5 0.0 -15.0
inter_unit_spacing 3.0
priority 1.0

Next we add the stats for the animal and all the other info to the new units details.
Spoiler Alert, click show to read: 

type AS Ballista
dictionary AS_Ballista ; Ballista
category handler
class heavy
voice_type Heavy
banner faction main_infantry
banner holy crusade
soldier Eastern_Captain, 12, 36, 1.2
animal wardogs
attributes sea_faring, hide_forest, hardy, can_withdraw, frighten_foot, mercenary_unit
formation 2.2, 2.2, 3, 3, 3, horde
stat_health 1, 0
stat_pri 19, 8, no, 0, 0, melee, melee_blade, piercing, knife, 25, 1.8
;stat_pri_ex 0, 0, 0
stat_pri_attr no
stat_sec 14, 4, no, 0, 0, melee, melee_simple, piercing, none, 25, 1
;stat_sec_ex 0, 0, 0
stat_sec_attr no
stat_pri_armour 5, 5, 0, leather
;stat_armour_ex 2, 0, 0, 0, 5, 0, 0, leather
stat_sec_armour 2, 1, flesh
stat_heat 1
stat_ground 0, 0, 0, 0
stat_mental 13, disciplined, highly_trained
stat_charge_dist 40
stat_fire_delay 0
stat_food 60, 300
stat_cost 1, 800, 150, 60, 140, 370, 1, 110
armour_ug_levels 1
armour_ug_models Eastern_Captain
ownership mongols, timurids, slave
era 0 mongols, timurids
;unit_info 7, 0, 7
recruit_priority_offset -50

So the category is handler, the animal is wardogs, the formation is more spread out than a regular unit, stat_sec are the animals attack stats with stat_sec_armour its
defensive stats.

Step 2: descr_animals
Now, according to descr_animals:-
animal class (current possibilities - wardog, pig)
So we must use one of these unless we want to create a new battle model. (I don't even own a 3d application, so thats a no go for me).
But there are no battle model for dogs or pigs, so we must change the info in descr_animals.


type wardog
class wardog
model wardog
radius 0.75
x_radius 0.35
height 1.0
mass 1.0


type wardog
class wardog
model mount_elephant
radius 0.75
x_radius 0.35
height 1.0
mass 1.0

Now, there is deffinitely a battlemodel for mount_elephant so we know this should work!
Here's the proof from testing...
Spoiler Alert, click show to read: 

Spoiler Alert, click show to read: 

Spoiler Alert, click show to read: 

Spoiler Alert, click show to read: 

Spoiler Alert, click show to read: 

The new unit behaves in exactly the same way as its forerunner in RTW did, all the info the game needs to make this work is already in place.
The only thing omited in M2TW was the actual models and anims for the dogs and pigs.
Any model maker with experience of creating new battlemodels from scratch should be able to put together a decent dog or pig or anything else they fancy for that matter and we will have a great new unit.
As someone who has no experience with models I must now wait and hope that someone with as much enthusiasm as myself will make the project complete. (Either that or I learn to use 3ds, :hmmm