Can this be pulled of biologically...maybe but probably not.
Could a non-Newtonian fluid be used in armor...yes.
Should you use an NNF in armor...probably not.
- The biology idea. Here's the problem with the idea. Fluids in any body be it plant or animal are a transfer medium for everything from energy to waste to nutrients. A NNF doesn't fulfill this function effectively as it doesn't really act like a fluid and would make a terrible transfer medium...basically its just too thick.
That being said (and I am not sure how the biology would work here) you could have a creature whose fat is actually a NNF contained within sacs under the skin. The NNF would have to be contained in all three dimensions otherwise it would ooze down to the bottom of the body. This would make the most sense in a polar/cold region as I would hazard a guess that NNFs are good insulators.
So you'd need some sort of gland...or pump or something in the creature that creates a NNF and pumps it into these sacs. Far fetched maybe but it could probably be done.
It simply won't work in plants... they wouldn't be able to function with a NNF
You could absolutely use a NNF in armor. Ignoring the biology side for a moment I would suggest you could have a hard casing (of the ballistic material of your choice) that is hollow and the cavity within is filled by a NNF.
I am not sure how good the stopping power of NNFs are...at least in the real world. It could be made feasible with a super durable imaginary NNF. One additional concern would be weight. Fluids are dense...obviously. More density is good BUT weight is also a consideration, heavier armor is bad for mobility and endurance.
In short you would have to design something that:
- Has better stopping power than modern ballistic defense systems
- Is strong enough to validate the added wegiht