The Plant Harpy is a fragile species that excels at inflicting status effects with their attacks, hitting their enemies with Poison, lust damage and slow.
The power of the unit comes from the status effects, hitting the target and then flying away while the enemy slowly dies while trying to catch up. For example, if an enemy has 2 movement speed and is standing 4 tiles away from your Plant Harpy, then the enemy can only run up to your Plant Harpy without attacking it. Since the Plant Harpy has Agile, this will in turn allow the Plant Harpy to attack (applying the status effects) and then fly to a safe distance. The enemy can't follow the Plant Harpy effectively because of the slow effect. By doing this, the Plant Harpy can take down enemies much tougher than itself.
That only works under ideal conditions however, such as a large enough field and that the enemy will only follow the Plant Harpy. If there are lots of enemies, the field is too small, or there are field-wide effects in place, they're a fragile unit you can't get much use out of. Their stats are lacking and their few skills are next to redundant. They're fragile despite their best stats being health and Strength, their poor lust growth makes their Pheromones a minor addition, and Slow has limited use. You could consider adding a Frost Spear on the Slow to instantly freeze enemies, or a LifeStealer with PoisonFlight to maximize the LifeSteal onAttack, but all in all the Plant Harpy is not a recommended unit.
Quite the nuisance to face. Try to take them out quickly.
Consider breeding/fusing a Plant Harpy with Blink/LongJump/Jump, to allow for a more effective hit and run play style. The enemy will once again have issues following the Plant Harpy, as they slowly die from Poison damage.
They love to fly up to their partner, shoving their genitals into their face and use their strong limbs to hold on. Because of their low weight this rarely causes problems.