So you want to throw a mid-level plant creature at the PCs. Which do you choose?
Both the shambling mound and the tendriculos are CR 6. The shambler is smarter and understands Common as well as Sylvan; that intelligence also leads to more skills. The tendriculos make up for its lack of gray matter (or green matter?) with numbers, being more likely to be found in a pair or grove of 3–6 individuals. The shambler is Large while the tendriculos is Huge…but apparently loosely packed, as they both weigh about the same. They are both carnivorous: the shambling mound constricting its victims and then feasting off the corpses, while the tendriculos can paralyze, swallow whole, and digest (likely thanks to its fungal components). The tendriculos is tougher in general, but the shambler more resistant to fire and famously (and alarmingly) able to feed off electricity.
In the final analysis, the tendriculos is slightly more dangerous—even taking into account the shambling mound’s intellect. In magical settings, lightning-scarred forests, and in coordination with other intelligent creatures, beware the shambler…but in most forest encounters, the “ravenous purple maw” (per the Bestiary 2) of the tendriculos is the thing to fear.
Locals call the bridge to Dorfin the Troll Bridge, as travelers often disappear from there without a trace. The guilty party is not a troll, but rather a tendriculos that clings to the side of the stonework like ivy.
The tendriculoses of the Miriani Jungle are not content to wait for prey to wander into their territory. Instead, they swing through the cathedral-like canopies of native spire trees, brachiating like apes by their tendrils as they hunt.
The hot spring at Jøter seems to be unguarded. But the thick carpets of lichen and moss that gather around the entrance are actually two specimens of a rare tundra tendriculos. The hungry plants are just intelligent enough to wait to strike victims as they are leaving the spring, rested and incautious.