You mean theres another kind of encounter???
Why don't people tell me these things.

No seriously not everything is random but I'm not much for planning ahead. Plus I tend to skim through books and find something cool and resolve to throw at it someone (not that I actually get oppurtunities to DM but I can pretend). To me the less something fits in the better usually.

Walking in the Desert? Flying Sharks.
On a Ship? Cranium Rats.
In modern times? Orcs.