A spin-off from something I, for one, was schooled on ina current thread where everyone is learning stuff about chickens.
And for poop and pee, as I learned, I think the definition of excretory functioning is not “one’s solid, one’s liquid” (see cited thread to learn about bird shit), but I don’t know what it should be as the end product(s) of differing metabolism frameworks as a zoological matter.
Poop is mostly undigested food residue (plus some products of blood breakdown). Pee is how nitrogenous waste is disposed of, as well as most miscellaneous substances that the body doesn’t have any other way of getting rid of.
Yes, the pee<–Nitrogen thing was pointed out by Colibri, but what I didn’t know (and still don’t) is about the “reasons” for “no other way” (a generalization I had no way to even make)–and by OP’ing animal kingdom I wanted to try to group the different classes of “the body.”
And if all animals share the same physiological descriptions for the reasons and body I’ll be mighty surprised (which wouldn’t be surprising).
I think, and could be wrong, that the only non fish vertebrates that do a combined function are all in the Class Reptilia
That would include Birds Snakes Lizards Turtles Crocodiles in living animals
Incorrect. The duckbill platypus, for example.
My understanding (which may or may not be accurate) is that separation of poop and pee is a mammalian thing (and not even true for 100% of mammals, see above example). Other animals have a “cloaca”, which is derived from an ancient word meaning “sewer”, which is the common outlet for kidneys, intestines, and, in the female, the reproductive tract.
OH, no fair!!
You had to bring in mother natures hold over bastard child 
Cloacal mammals besides the platypus: echidna, some shrews, tenrecs, golden moles, and marsupial moles
If we’re trying to answer “what percentage of animal species…”, then facts about mammals (while interesting) are basically a rounding error. There are about 1.2 million known animal species, and the vast majority (almost a million) are insects. There are only about 50k known vertebrates.
Given that it’s easier to find larger animals, it’s likely that even more of the unknown animal species are insects or other very small animals.
According to a brief internet search, insects poop through their anus, but excrete nitrogenous waste through something called malpighian tubes. Maybe someone with more insect knowledge can clarify, but it looks like the answer to the OP’s question is well in excess of 90%.
So why is nitrogen so special a waste product?
Ammonia, which is a nitrogen compound, is an end product of some metabolic processes.
It is also very toxic so there are mechanisms to package it up straight away as other more stable nitrogenous substances like urea and ship it out in urine.
It’s hard to use ammonia metabolically, whereas most other ‘waste’ metabolic compounds can be broken down or re-used by animals. But pee isn’t a special pathway that ammonia takes while other metabolic wastes go somewhere else; they all go to pee.
Poop is just food that wasn’t absorbed – it’s stuff that never got through the skin (including the skin lining the guts) and was never really part of the body.
Pee, on the other hand, is stuff removed from the bloodstream, which means all the waste from metabolic processes inside the body (except carbon dioxide which is exhaled). Again, animals can deal with most of their metabolic waste processes, but not all. By volume, the major one they can’t deal with is ammonia, so that’s the main component of pee (besides water).
So internally, pee and poop are completely different things. It’s just chance/convenience/efficiency that makes some animals discharge them from basically the same spot.
As an aside, while true, that’s a very big “except”. The lungs are, by a considerable margin, the primary excretory organs in the body. They’re also responsible for disposing of a great deal of water (also a waste product for animals), though I’m not certain that they outpace the kidneys in that regard.
Back to the kidneys, it really wouldn’t be possible to have a separate process for removal of every possible material the body wanted to get rid of, because there’s a near-infinite number of possible materials that could potentially get into a body. At some point, you’re going to have to have an organ for getting rid of the miscellaneous everything-else, and it happens that that’s the kidneys. They accomplish this everything-else task by effectively using a whitelist, not a blacklist: They don’t remove the foreign substances from the blood; they remove everything from the blood, and then put back the stuff that they know is supposed to be there.