I’ve a basic zoology query about terminology. What is it called when the creature such as fish and birds excrete the feces and urine from the same orifice?
Well, the orifice is called the cloaca. Is that what you are asking?
Close but not exactly however. Is there a categorical term for animals that have a cloaca? Is there a taxonomic category for fish, reptiles and birds?
I doubt it. Not a “real” taxonomic category, anyway. Anything that includes fish along with reptiles and birds is going to include mammals too. See here (with added erroneous XKCD).
Unsurprisingly, it’s “excretion.” “Excreta” include both feces and urine. We often use excretion as a synonym for defecation, but this is not strictly correct.
No. In any case, neither “fish” nor “reptiles” are generally recognized as formal taxonomic categories these days. You would just have to say non-mammalian vertebrates (and even this doesn’t include the monotremes, which have cloacas).