OK, downsizing the mastodon to 9 HD and removing the gore attack yields...
Huge Animal
Hit Dice: 9d8+54 (94 hp)
Initiative: +0
Speed: 40 ft. (8 squares)
Armor Class: 16 (-2 size, +8 natural), touch 8, flat-footed 16
Base Attack/Grapple: +6/+25
Attack: Slam +15 melee (2d8+11)
Full Attack: Slam +15 melee (2d8+11) and 2 stamps +10 melee (2d6+5)
Space/Reach: 15 ft./10 ft.
Special Attacks: Trample 2d12+16
Special Qualities: Low-light vision, scent
Saves: Fort +12, Ref +6, Will +5
Abilities: Str 32, Dex 11, Con 23, Int 2, Wis 10, Cha 5
Skills: Listen +8, Spot +8
Feats: Alertness, Endurance, Improved Natural Attack (slam), Iron Will
Environment: Temperate swamps
Organization: Solitary, pair, or herd (3-6)
Challenge Rating: x
Treasure: None
Alignment: Always neutral
Advancement: 10-27 HD (Huge)
Level Adjustment: -
Trample (Ex): Reflex half DC 25. The save DC is Strength-based.
I dropped Improved Natural Attack (gore) and Toughness.
It looks like it had a swim speed which converts to 15 feet. Do we want to stick with that, or simply give it a racial bonus on Swim checks?
Do we want to give them any special ability related to shoveling?