These are the options for all three creatures:
Spike tail - Adds 1 tail attack.
Icy breath - Gives the Thundertusk a strength 4 breath attack, that can not be used in the same turn as its other shooting attack.
Can be taken in packs of 2 - 8.
Vanguard - Gives them the vanguard special rule.
Thickhide - Gives them the scaly skin (5+) special rule.
Jaws of iron - Gives the Stonehorn a chomp attack with the multiple wounds (D3) special rule.
Mace tail - Gives the Stonehorn 1 tail attack at +1 strength.
The Thundertusk and Stonehorn are both cheaper than thier Army book equivilants too, probably because they do not have any riders.