So in my 'headcanon' for the bloons series, the reason monkeys are so threatened by simple hovering balls has to do with all the terrifying things they are capable of.
First off, yes, they are sentient... things... Whether this is just a form they take to enter the monkey world is another question. They have a range of psychic powers, including telepathy (passing information between eacother and reading the minds of monkeys), telekinesis (which is actually how they float, regardless of wind), and when close enough to a monkey, they can begin to mentally manipulate them.

And thats how we get to this point, a 'what if' pic of the Bloon Killer Squad overtaken. Minus Wrigley cuz reasons, in fact he might be the only one who could save them from this fate.
This could also explain how/why monkeys like Psi are learning the ways of psionics, to wage war with bloons on their own terms.