Okay, first off, most of this stuff is taken from this thread. Secondly, I thought of these things up according to Nick's Comment that any Plant Monster(s), if introduced would have to "stand out, or just not have them be an entirely new, fleshed out monster family. Plant monsters as ancillary threats could work." I took this to mean that the Plant Monster should a) Be a striking new Monster Family, or b) a few mundane monsters, alternatively, they could c) ancillary (secondary/assisting) threats.
I chose to take option a and c. Creating a few ancillary threats which are intended to be striking whilst remaining just that, ancillary.
And here they are, all two of them!
Anyway, this is how I envision the Plant Monster working.
The Plant Monster works along the same principle as the Rock Jelly i.e. it has a x% chance of spawning in place of a rock. Except the Plant Monster has an x% of spawning in place of a bush. When it's spawned it appears, for all intents and purposes, as a normal bush; except it can't be destroyed. No projectile, no explosive, no charge attack, no vial, no anything will destroy it whilst it is still a bush. But other than that it appears as a normal bush, all ready to be cut down for Crowns.
But when you get near to the faux-bush it turns into the monster illustrated in the link above (I think using the code which makes monsters attack when a Knight is near would be suitable, just replace the attack line with the transform animation), the head and legs pops out and then the bush starts scuttling towards the nearest knight. It can now be treated as a normal monster.
{{A semi-unique feature for this monster is that it's bush can absorb y amount of hits of any kind (maybe scaled to tier?), and then it loses the bush, becoming denuded. Denuded Plant Monster
[If this seems unnecessary then you can ignore this 'feature'.]}}
Movement Behaviour
The monster quickly scuttles after the nearest knight and periodically unleashes projectile attacks regardless of current situation to keep the knights on their toes/give the Knights a chance to attack it.
Attack Behaviour
The attack animation (drawing over here) is where the monster brings its neck back in preparation for the attack. A short time spent preparing means that it's a melee attack, whereas a long wind-up equates to a projectile attack. The projectile attack is 1-3 balls of green similar to a Gun Puppy attack, the melee attack is just a quick bite forward.
Note: I think that this creature needs no tier-specific skin. Just treat like a Rock Jelly; same set of skills, just levelled with the stratum.
I hope that this fulfils the idea of a plant-based ancillary enemy, Nick?
Spawning Behaviour
The Vine Monster is something which I envision spawning in large areas. The core pops out of the ground and then the vines spread out from the center by a pre-determined amount.
Attack Behaviour
The monster's - only - attack would be where a blast originates from the center of the core, and proceeds to spread outwards to the tips of the vines, where it will disappear. The attack can either be a standard 'do a certain type/amount of damage', but I'd rather that it be status-inflicting, with the Status effect covering the Core to indicate what status it is going to inflict. The pulse is periodic with a large enough gap for Knights to get close.
Although Gunners could just stand far away and unload into it, there maybe should be a retaliation attack where hitting it with a projectile - and perhaps a bomb? - could unleash a 360 burst of projectiles.
Notes
The Core can either be a simple 'hit enough, it disappears' type of enemy, or it can just go into hibernation where the vines stay put, but the Core disappears for a bit; the latter would make it annoying, the former would just make it a throw-away enemy, won't truly know until it's been tested. Also, making an invulnerable version would make it it into the ultimate enviromental hazard.
There is something called the Dark Harvest Festival