How big is the fruit?
I think this is a Malaysian Lemon. Perhaps unripe, perhaps a hybrid with some other citrus (lime?).
Lemons generally don't do well in truly tropical conditions, where they tend to be replaced by limes.
In Malaysia there is a variety grown in the highland areas which tends to be almost cylindrical, rather like your fruit, although not exactly like your picture. However, the same variety can look very different when grown in different climatic zones.
All the photos below are from web-sites about seedless Malaysian lemons.
What do you think?
Mike aka Citrange