What's wrong with mosquitos? (And recent historical experience suggests that a mysterious creature called the "insurgent", created by mysterious means in the mysterious Orient, is plenty dangerous to modern armies.)

@AlexP >:(⠀⠀⠀⠀⠀

Are you asking a worldbuilding question? Because by definition, "no, it isn't possible" because humanity doesn't have that kind of gene editing abilities. Please remember, we don't answer questions about real life. Consequently, "is this possible" is usually the worst way you can ask a question here. What worldbuilding problem are you trying to solve? Are we to ignore what genetics we do understand and simply try to create a fictitious creature? What are our limits (limits are required, see help center)? Are we fighting pistols? Howitzers? Nukes? (continued)

...Let me make a case in point. If I genetically create a creature that can only be stopped with a nuclear weapon, and then give that creature enough intelligence to know it must stay within a heavily populated area so that the politics of the military will prevent the nukes from being used, have I met your expectations? If not, exactly why not?

@JoinJBHonCodidact i just want something to live in caves & is able to cause serious pain in the assery to developed nations. It doesn't need to be able to cause collapse, just be a threat

You need to include useful limitations like that in your Q. We need to know what we can, and cannot do. We need to know, as exactly as you can, what "be dangerous" means. I say this because right now there's not a lot separating a giant gas-passing sloth from a swarm of flies. After all, you haven't told us what kind of damage you're expecting or where you're expecting it. It would help to know exactly which military and exactly where combat is expected so we can identify environmental limits. Something that affects a navy is much different than an air force or the infantry.

@AlexP I'd like to see a mosquito that can take out a tank (air filters???). Come to that , the urban environments round here are barely big enough for cars let alone a tank, how are you going to get a creature big enough/fast enough to fit the bill OP (generally - the bigger : the slower.

@ARogueAnt. >It can be multiple different species working together, it doesn't have to just be one creature.

Even a herd of elephants isn't going to present much of a barrier to a tank are they, unless the elephants are trained to use high-explosives/anti-tank weapons. Hm, maybe that's what you're after?

@ARogueAnt. Nah, think smaller. I know OT is looking for something like Godzilla, but he's not defined what he's looking for. I want to engineer a lichen that thrives on the chlorination and/or fluoridation in developed nations' water supplies and emits as a byproduct a poison - just enough to give everyone the trots. It just happens to like attaching itself to metal pipes. It would bring a nation to its knees and no military could defeat it.

@ARogueAnt. if you can get on top then break all the ̷m̷c̷ ̷w̷i̷n̷d̷o̷w̷s̷ sensors or break open a hatch & get in that tank is going to be dealt with.

modern tanks can be sealed, so there is not animal that can bother people inside them and as long as a the tank is running nothing will bother the tank itself. sometimes the answer is its not possible.

@John if you can get on top then break all the ̷m̷c̷ ̷w̷i̷n̷d̷o̷w̷s̷ sensors or break open a hatch & get in that tank is going to be dealt with.

You basically seem to be looking for King Kong level strength to do what you say, take a look at the answers here. Again, not possible or suited to urban environments.

@john then then break all the ̷m̷c̷ ̷w̷i̷n̷d̷o̷w̷s̷ sensors, they're only protected by some glass at best

Sensors are not protected by glass, they are designed to withstand small arms fire and explosions, no animal is going to bother them either. any animal large enough to even risk damaging a tank sensors will be big and slow enough the tank can just run it over. you should study up on modern tanks.

@OT-64SKOT Why don't you write an answer, that's perfectly acceptable to do on worldbuidling.

@John you dont need to kill the tank or crew, just the supply lines and personel that keeps them going. Although a sufficiently intelligent creature could hide and attack a tank crew when they stop for a break, or perhaps strike at them when they drive around with open hatches before they know they are under attack.

@Demigan the OP specifies that it needs to be a threat to a tank. And making an and training an intelligent creature is way beyond simple genetic engineering. By the time we can do that we probably won't be using tanks or human soldiers anymore.

@John "it needs to somehow provide a threat to a tank". Killing the crew or support structure that enables tanks is a threat to them.