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It's a valid choice.
Referencing the material the source is said to be from, in that book it is a specific monster, however that monster is based on a creature that's pre-existing with a line indicating the material it is from [Male two-headed troll (Tome of Horrors Revised 353)]. This indicated that the creature is from page 353 of Tomb of Horrors Revised (a 3.5 supplement). However, given that the only ToH Revised for 3.5 I know of doesn't mention troll and is only 36 pages long, I'm unsure of what they were actually referencing.
In anycase, the creature is not a template, nor a unique creature, and is therefore valid to take. However, I am unaware whether PFS restricts sources for wild shape and the like, and thus whether this is PFS legal.
Tome of horrors, not Tomb.A third part product, so I don't think it is PFS legal.
Maybe if the player has Pathfinder 32: Rivers Run Red, where the stat block is published, but I doubt it as it is only a stat block, not a complete monster entry.
Only Bestiaries 1 through 6 are listed as legal sources for polymorph effects in PFS. The only things legal from Rivers Run Red are the spell tracking mark and the customized summon list.