My proposal for 6 (or higher) lvl Druid Animal Companion:
http://www.d20srd.org/srd/monsters/wolverine.htm witch Battle beast Template :]
Why?
Average wilverine have 3 HD witch Template he have +1HD and when Become the 6 LVL Druid Animal Companion he have extra 2 HD
So 3+1+2= 6HD
And when Wolverine have more then 4HD he become Large. So our Druid can ride on him into the Battle :]
Now because of Battle Beast template our Wolverine can wear All Armours :] But i think that Chain Shirt will be Fine (Cost only 400gp)
So our 6 HD wolverine will have that Stat:
Str: 14+3(Temlapte)+8(Size)+1(Animal Companion)=26
Dex: 15 -2(Size)+1(Animal Companion)=14
Con: 19+3(template)+4(Size)=26
Int: 1
Wis 12+2(template)=14
Cha: 10
AC: 10+2(dex)+2(Natural)+2(size)+2(Animal Companion)-1(Size)=17 +4(Chain Shirt)=21
Attack: 2 Claws +11 and Bite 6
Damage
Claws D6+8
Bite d8+4
Speed 30+10(Template)
Feats: Rage, Scent and Dodge.
3 Attack Two witch +11 and one +6
Damage from 9-14 and 5-12
AC like Warrior in Mitril Full Plate or Ranger witch Mitril Chain Shirt.
Add to that. Druid is Riding him like Horse. So what the Problem Casting Long Step and Oak Body?
Fist have Duration 6 hours and give +10 foot Speed.
Second give only for 1 hour +3 to Natural Armour.
if You have a 9 lvl Druid get iven Better. Because our Battle Wolverine get +2HD and +2 Natural Armour and +1 to Str+Dex
20 lvl Druid have bi his site a 14HD Battle Wolverine. Witch AC 10+4(dex)+14(Natural Armour)=28
Str 30 plus Rage...
For my its Monster. I'm not Sure but if you could combine Templates than Battle Best (training) + Magebred(Born) will create a Biggest Monster.