Looking through the list, if we use the 6 HD rat swarm we've got one sort of monster for every HD from 1/2 to 8 with the exception of 7 HD. Now I fancy filling the gap. From the SRD there are the following candidates:
Bison, Dire Ape, Dire Wolverine, Giant Wasp, Lion, Medusa, Owlbear.
They're all Large except for the Medusa, who would be an interesting sight - wouldn't her snaky hair get flaming bites as well? Although there is something to be said for a Large monster, just in case we have Huge carnivorous walls. If we do, I fancy the Dire Ape best.
Which do you prefer?
EDIT: Here are their stats, the Dire Ape version is a lot tougher than the medusa. The size Large and the Str 24 increase its damage-dealing capacity considerably and it gains a Contruct's immunities and bonus HPs, plus I left in the Rend special attack which the template as written would remove. It's more of a CR 4 than the CR 3 the template gives. Well, that's par for the course for templates that don't include a CR adjustment for size, maybe I should add such a clause to the template.
Dire Ape Burning Zombie
Type: Large Construct
Hit Dice: 7d10+33 (71 hp)
Initiative: +1
Speed: 20 ft. (4 squares), climb 10 ft.
Armor Class: 19 (-1 size, +1 Dex, +9 natural), touch 10, flat-footed 18
Base Attack/Grapple: +4/+15
Attack: Bite +11 melee (1d8+7 plus 2d6 fire)
Full Attack: Bite +11 melee (1d8+7 plus 2d6 fire) and 2 claws +6 melee (1d6+3)
Space/Reach: 10 ft./10 ft.
Special Attacks: Flaming bite, rend
Special Qualities: Damage reduction 5/slashing, darkvision 60 ft., construct traits
Saves: Fort +2, Ref +3, Will +2
Abilities: Str 24, Dex 13, Con—, Int—, Wis 10, Cha 1
Feats: Toughness
Skills: —
Environment: Any land or underground
Challenge Rating: 3 [more like a 4?]
Alignment: Always neutral
Rend (Ex)
A dire ape burning zombie that hits with both claw attacks latches onto the opponent’s body and tears the flesh. This attack automatically deals an extra 2d6+10 points of damage.
Medusa Burning Zombie
Type: Medium Construct
Hit Dice: 7d10+23 (61 hp)
Initiative: +1
Speed: 20 ft. (4 squares)
Armor Class: 18 (+1 Dex, +7 natural), touch 11, flat-footed 17
Base Attack/Grapple: +4/+4
Attack: Bite +5 melee (1d6+1 plus 1d6 fire) or shortbow +5 ranged (1d6/×3) or dagger +5 melee (1d4+1/19–20) or snakes +5 melee (1d4 plus 1d6 fire)
Full Attack: Bite +5 melee (1d6+1 plus 1d6 fire) and dagger +0 melee (1d4/19–20) and snakes +0 melee (1d4 plus 1d6 fire) or shortbow +5 ranged (1d6/×3)
Space/Reach: 5 ft./5 ft.
Special Attacks: Flaming bite
Special Qualities: Damage reduction 5/slashing, darkvision 60 ft., construct traits
Saves: Fort +2, Ref +3, Will +2
Abilities: Str 12, Dex 13, Con—, Int—, Wis 10, Cha 1
Feats: Toughness
Skills: —
Environment: Any land or underground
Challenge Rating: 3
Alignment: Always neutral