In scenario I am talking about there are civilizations of primitive sentient aliens, and they have acces to big (able to fly with ~100 kg of cargo including passengers) flying creatures which have been tamed and are used similarly to planes during WWI - for reconnaissance, dropping explosives (nothing fancy, nutlike shells filled with shards of rock and gunpowder), taking down enemy units (aliens weigh 40 kg at most so there can be two+bunch of cargo like arrows, bows and such) and military dropoffs (parachutes made of giant leaves) and question is: what steps could they take to fight enemy aircraft excluding air to air combat, and - if you know - how could they redesign it to fight human WWII grade planes.
Your viable AA will be on par with the siege engines of the roman empire era. You will have bows & arrows and catapults / ballistas. You would load your catapult with grape shot or nets, and your ballista with tethered harpoons or a sabot full of smaller darts.
If you can light your projectiles on fire, that would have the added effect of frightening the flying animal. If your tribes can make a reliable fuse, then they could launch the same time of explosive shell to emulate modern flak AA. That would be super risky if the fuse wasn't reliable, since you might damage your AA battery.
Smoke screens could also act as an area denial and limit the enemy's targeting capabilities, but now your area is smoky as well.
Chances are the enemies will be able to fly higher than your catapults can reach, which won't be more than a few hundred meters max, depending on the material tech they have access to.
The enemy could simply hover over your houses / batteries and drop incendiary (pots of oil) until your village is smoldering rubble. Prevent that by building underground or out of stone, and consider concealing your bases with foliage to avoid detection.
Crossbows, bows, catapults loaded with several dozen fist-sized rocks, using chemical fires with lots of smoke that slowly rises to make approaches hard or force them to land/fly away, hot-air balloons anchored to the ground with nets inbetween (between the ropes, not the balloons) as flying hazards. Streets and roofs with ropes/nets inbetween to catch landings and potential large ammunitions, those spikes on the roofs like we use against doves and other birds only more solid to harm anyone that dares land. Wooden roofs over important open area's and last but not least train men in those roman shield-roof maneuvers.