I didn't know what these were until I started geocaching. It's called a bison tube aka (unofficially) the mini scuba tank. According to Cacheopedia.com, it's called a bison tube because it was originally made by Bison Industries, not because it bears any resemblance to an actual bison. I have to admit, the first time I saw bison tube in a cache description I expected it to be brown and furry...needless to say I logged a dnf for that one.
These little micros are versatile. I've seen them hanging from fences, signs, trees, in light poles, and at the base of signs. Tubes like this are naturally camo'd for blending in with metal. Wrap them in camo tape and suddenly they are camo'd for a bush. And, they're watertight.
It is extremely important to note that just because the description says you're looking for a bison tube, you might not really be looking for a bison tube. Some cache hiders camo their bison tubes with wood, moss, and other natural materials. While the cache itself is a micro, it could be permanently affixed to camo the size of an ammo can.