This is a type of tropical bat – an unusual subject for an artist. The one painted by Van Gogh was stuffed and mounted. Just look at its stiff, unnatural pose.
Van Gogh knew a man in Eindhoven (NL), Antoon Hermans, with a collection of more than 300 mounted exotic animals. This bat may have come from that collection. Van Gogh wanted to depict the translucent wings clearly, so he placed a light source behind them.