But "it" is for inanimate objects

Not quite. "It" is a general reference pronoun with a function akin to "the". It can be used to refer to anything that is a thing, even if said thing is animate and/or living.

When referring indiscriminately to a specimen of fauna, "it" is a linguistically appropriate identifier whereas "they" would only really be entirely appropriate when referring to an individual or subset of individuals, regardless of species or animacy.

Since this fish has no distinguishable identity apart from the cultural impact it may spawn, I reckon it's more appropriate to use "it" but "they" could also work.

I am not a linguist. But if you are, feel free to correct me. If you feel like pretending to be a linguist, go talk to an LLM cause IDC.