Snatcher

Nothing summons it and it does not wander in. It spawns already fixed to a block of surface jungle grass and stays anchored there for its whole life. The detail that catches people is that the biome is not the requirement, the grass is, so a stray patch that spread outside the jungle proper can be hosting one, and so can jungle grass you planted yourself near a base. Digging out the block it is attached to does not get rid of it either.

It cycles between two states with no tell at all. Most of the time it works to a shorter maximum length, then for a couple of seconds it switches to a noticeably longer one, and it flips back and forth on a timer that has nothing to do with what you are doing. Judging your safe distance from the last time it lunged is the most common way to eat a hit from one. Either stay clear of the long version or do not walk past it at all. The stem also passes through blocks, so a wall of dirt between you and it is not cover.

Only the pincher on the end exists as far as the game is concerned. You can walk straight through the length of the stem without taking a scratch, and your own attacks pass through it without registering, so swinging at the middle of the vine is a wasted hit. Damage dealt and damage taken both live in the head. It is immune to knockback as well, so you cannot shove it back out of range once it has committed to a lunge.

The underground jungle grows the Man Eater, which is the same plant with roughly double the reach and a browner, red tinged head. That one drops Vines and ancient cobalt gear. This one drops neither, so if either is what you came for, the surface jungle is the wrong place to be looking.

Set one on fire with a weapon that applies a burn and it thrashes around while the damage ticks, dragging its own light with it. In a dark jungle that will occasionally light up a cave mouth you would otherwise have walked straight past.