I imagine them to run into a young Warthog.
"It is harmless as it is Ugly"
Famous last words, when mother Warthog thinks it has to protect its young...
a colleague of mine walked along the mountainside with her german dachshund, when a cow thought the dog was a threat to its calf.
she was so stupid to take the dog up in her arms to protect it.
her injuries were such that she never returned to work before retiring a few jears later.
that was ONE cow.
birds that live in great flocks can easily take down an aircraft, if the aliens use aircraft that have air intakes they could have a problem.
Quote:
...and then the evil called bugs...
large swarms of bugs could explode in theory, like a cloud of coal dust. if the aliens try to defend against a swarm of moskitoes with energy weapons they might create a bug-explotion.