As a general statement, which is more natural, #1 or #2?

1. An elephant has a long trunk.
2. Elephants have long trunks.

First, I thought of #1, but I'm not sure.

 

I'd say 2 is more common, but it would depend on the context. If a professor is lecturing about this topic, s/he might even say "The elephant has a long trunk" ("the elephant" meaning elephants in general, not a specific elephant).

 

As you're talking generally it's a lot better to use the plural. (2.)

[Edit] : I agree Franzi, not sure if you know who David Attenborough is, but it sounds very typical of one of his nature documentaries!

But I still think 2. is by far, more natural.