lol randommmmmm.
fish arent born pregnant (the only application would be livebearers, and i know that isnt true) Egglayers are out of the question. they are born, they mature to sexual age, they mate and release gametes into the water. no pregnancy.
Livebearers are born, they mature to sexual age, they mate once and the female can store sperm for as many as 6-8 broods from the same male (even if he is not around). after the stored sperm runs out, the female has to mate with a male again.
This is the extent of all common aquarium fish circulating in out hobby.
There can be a very rare and unusual fish species that is born pregnant (and possibly only born as female) but these would not be commonly discussed on a fish forum like this.
I am disregarding that one species of molly, the amazon, that is only female, because they are not born pregnant, they need a male of some other species in order to mate.
The only thing that comes to mind as being born a female and not needing males to reproduce, is most Rotifer species. but even those will sometimes make male individuals...But i doubt even these are literally born pregnant, the female rotifers need to mature before they start releasing eggs.
Theres also aphids. thats kinda what youre talking about, Thyra, but idk of any fish that behaves like aphids .___.