BOZ said:As implied by their name, these creatures appear as large, single-span stone arch bridges, complete with rails, posts and so forth. The type of stone, stains, vine covering, and other details will be appropriate for the local environment (see below). These creatures are extremely difficult to differentiate from true bridges. Only dwarves, or persons with engineering or stonemason talents, may be able to ascertain the identity of a spanner. In this instance, a successful Intelligence or appropriate nonweapon proficiency check at a -6 penalty is required before the true nature of the bridge may be seen.
BOZ said:Combat: Spanners prefer not to attack, if at all possible. They are benign, even friendly, preferring to gossip and gather information rather than attack,
BOZ said:Being curious and intelligent creatures, they will tolerate a fair amount of abuse as long as the creatures around them are talking. However, if irked, or crossed without permission, spanners are malicious and have no mercy. It the victim is near the edge of the creature (within five feet of a rail), the spanner will attempt to pitch him/her off the side;
BOZ said:otherwise, the spanner opens a hole through itself and under the victim. In either instance, a Dexterity check at -4 is necessary to avoid being removed from the bridge. The damage from this attack will depend on the distance from the bridge to the land or water below (1d6 damage per 10' of failing, up 20d6 maximum, with additional potential for drowning if the creature is spanning a waterway).
BOZ said:These creatures locate potential enemies by detecting their weight on its surface, or by feeling their vibrations in the ground up to 150' away.
BOZ said:Spanners are also capable of using a stoneskin spell-like ability once per day, which protects them from 8 attacks.
Shade said:First off, I'd up the HD to at least 24. According to the Making Monsters chapter in the MM, the minimum HD for a Colossal creature is 24.
Camouflage (Ex): A creature must make a successful Knowledge (architecture or engineering) check (DC X) to notice that a spanner is anything other than a true bridge. Dwarves may add their stonecunning bonus to this check. An untrained Knowledge check is simply an Intelligence check.
Skill ranks in Diplomacy and Gather Information?
The Awesome Blow feat could account for this. Or, borrowed from the xerfilstyx...
Toss (Ex): After a successful attack, a spanner gets a free trip attempt. If the spanner wins the opposed Strength check during the trip attempt, instead of simply knocking the opponent prone, it tosses the opponent into the air, dropping the opponent prone into any square it threatens. This toss deals falling damage as normal for the height in addition to the damage that was done with the initital attack.
Open Hole (Su): As a standard action, a spanner may open a hole in itself, up to 20 ft by 20 ft. Any creature in this area must succeed at a DC X Reflex save or fall, suffering falling damage as normal for the height.
Tremorsense 150 ft.?
Stoneskin (Sp): Once per day, a spanner can use stoneskin (caster level 8th).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.