There’s actually not much for me to say about the shambling mound. It’s a classic, to say the least—it dates back to TSR’s The Strategic Review #3 from 1975, which means it puts almost every other monster’s pedigree to shame. Shamblers have been used in any number of creative ways in published adventures—as animate leaf monsters, kelp piles, refuse heaps, elf slayers, alien vegetation, etc., etc. Want feats and variant monsters? Check out Dungeon Denizens Revisited for shamblers that spit lightning, spew spores, or release centipede swarms at death. Shambling mounds are in that monster sweet spot where they have been done well and in many ways but without being done to death.
All I have to add, then, are these simple thoughts: They are smarter than any clump of vegetation has any right to be. They understand Sylvan. And they crave elf flesh, which is weird for any plant creature. And weird means adventure opportunity…
A chance mutation breeds albino shocker lizards with exceedingly sharp teeth, but that are vulnerable to electricity. To compensate, they live in symbiotic relationships with shambling mounds—the mounds feed on their leavings and protect them from the electrical attacks of their kin, and the lizards attack any creature that threatens their host.
A centaur druid uses shambling mound allies to drive “two-legs” out of his forest, bolstering them with call lightning. But as the mounds get larger they get more aggressive, hungrier for humanoid flesh, and hooked on the electrical charge—a very literal and deadly buzz. After the shamblers wipe out a wild elf encampment the centaur never intended for them to trouble with, he tries to cut them off…only to have the lightning-emboldened Plant creatures take him prisoner to continue feeding their addictions.
Elves do not talk about where they came from…other than to obliquely refer to it as the First Handsome Home. In actuality, their world was this world, only unbent. On the verge of losing their nation, their minds, and their very race itself to the Adversary, a plantlike ancient evil, elf archmages bent the flat world into a globe in order to contain It. The sprouting of shambling mounds millennia after this event is the first sign that the Adversary stirs in Its prison…
Dungeon Denizens Revisited 58–63 & Pathfinder Bestiary 246
As mentioned above, Jason Nelson’s chapter on shambling mounds in DDR has lots more info on shamblers, including more feats, variant creatures, and their planetary origins far from Golarion.