They're envisaged as a challenge for low to mid expert level adventurers (4-10 or so), whereas for low level parties they're absolutely terrifying, and at high level they're just a nuissance.
Thought I might start posting some critters out of my notepads. Some good, some wacky, some written when I was a kid and really, really odd.
But many based on things in nature. Such as...
Scorpion Fly
by Cab DavidsonAC: 5
HD: 5*
Movement(flying): 180’(60’)
(walking): 90’(30’)
Attacks: 1 bite/1 sting
Damage: 1d4/1d4+poison
Number Appearing: 1d6 (1d6)
Save As: Fighter 3
Morale: 9
Treasure Type: V
Intelligence: 1
Alignment: Neutral
XPV: 300
A slender, 6’ long flying insect with a striped body, somewhat beak like proboscis and a long, curved tail reminiscent of that of a scorpion. It can attack with both its bite and its tail, a successful strike from the tail requiring a save vs. poison or the victim will die in 2d4 rounds. Typically a predator in open woodlands and pastures, the scorpion fly will also enter caves and can be found a surprising distance into dungeons, where they hunt and devour smaller humanoid species.