Some horror themes/ideas that I think work well with jungles:
* Carnivorous and/or parasitic planes; entangling vines and creepers
Example: A gaunt, disheveled humanoid crawls out of the foliage, covered with short vine-like growths sprouting from large percentages of its flesh; and apparently suffering hideous agony. It croaks for help in a raspy voice. Anyone touching or attempting to treat the individual might run the risk of contracting whatever has caused the infection. Or something large and nasty nearby may have implanted whatever it is.
* Terrible diseases, poisons, parasites - often carried by small organisms like insects, small plants, frogs, worms, snakes, and/or rodents.
Example: Anyone not having spent significant amounts of time in the area has a 1 in 10 chance of developing dysentery each time they eat something grown or killed from the region. While suffering dysentery, victims are unable to keep nutrients inside of them for various reasons. Victims suffer one level of exhaustion that cannot be removed until they recover from the disease. Furthermore, victims regain no more than 1 hit point per hit die spent to heal, and regain no more than 1 hp per level or hit die per long rest by natural healing. And finally, each day during which a victim suffers from the disease counts as one in which the victim has not eaten - per the rules on starvation. To recover from the illness, the victim may make a con save each time they finish a long rest; they need to make two or three con saves (DC: 12 maybe?) to recover.
* Ambush predators, monsters, enemies
Example: Classic jaguar ambushes a PC who wanders off alone. Or possibly something more fearsome.
* Waterways filled with dangerous and usually unseen carnivores
Examples: Crocodiles, piranhas, etc
* Cannibalism in practice, curse, or harmful and dangerous superstition
Example: PCs encounter a clearing with a 10-foot tall statue of a crocodile-headed human, depicted as holding two smaller, humanoid figures in clawed hands. At the foot of the altar is a jagged rock covered with ruddy-stains. Various bones have been left - as offerings, decorations, or incidental leavings around the area. Any PC who touches, investigates, or otherwise comes in close contact with the statue begins to suffer from cannibalistic urges and ideation (i.e. gradually begins thinking about eating people),
* Isolation and claustrophobia due to environment (e.g. isolating mists, thick and/or mostly impassable foliage pressing in, terrain that obscures any references for direction or navigation)