I don't think such an ending even exists in Polish, it would turn to "mali", when in masculine personal plural. Masculine personal plural is for groups of people that have at least one man in the group.
But this is the "not masculine-personal" (sometimes referred to as 'feminine' plural): it takes every noun that was not put in the masculine personal group. Even if the noun in singular was masculine (which is not the case here, 'cause 'zwierzę' is neuter anyway).
It is strange that the idea that 'fishes' can only refer to species of fish has taken hold. This seems to be a recent change. When I was at school we were definitely taught that fishes is just an alternative plural. The two forms are used interchangably in the King James Bible. Though I guess its not so common these days to get that read to you every morning.
It's actually definitely not the case that 'fishes' is only used for species of fish. I just checked the British National Corpus and there are loads and loads of examples of "fishes" being a simple plural in contemporary English. I haven't counted but it looks like this is actually a bit more common that using "fishes" to refer to species of fish. Here are some examples:
"One surfaces nearby with several small fishes dangling from its beak."
"We could do with some loaves and fishes."
"I felt about me; and my hands came in contact with several fishes."
It is true that "fish" is hardly ever used to refer to multiple species of fish but "fishes" can be used either for multiple species or simply more than one individual fish.
Is is not so reliable to get subtle grammar rules from the internet. Often someone will come up with something without looking properly at the evidence and this will get copied onto lots of different web pages so it seems like it is standard. I notice that even the Oxford English Dictionary now makes this same claim about "fishes".
I think the confusion has arisen because of the way that mass nouns such as "wine" or "cheese" are used. It is true that in English "wines" and "cheeses" always refer to several kinds of wine or cheese. But "fish" is ambiguous between either being a count noun or a mass noun, and the count noun has both "fish" and "fishes" as alternative plurals.
Well that's enough said about "fishes" for the time being. :)