It's a molly, not sure on specific kind but I'm guessing a black molly cross since it looks small and most mollys are rather big compared to the black mollys. I don't think it is pregenant, and the round fin just means it's a female. And if she is the only female in the tank I reccomend getting some more girls or else the males will probably wear her out.
I would say that's a younger female molly for sure. Mostly black (obviously) with perhapse a bit of marble in its line. Could be preg, but looks overfed to me. I love mollies and have short tail and lyre tail of marble, black, silver, and dalmation.... and how could I forget my gold dust molly.