About halfway through the Deeprun Tram, at the bottom section of glass tubes, you can glimpse a large monster named "Nessy." This is a reference and nickname for the legendary Loch Ness monster, which the World of Warcraft monster resembles.
Because the Deeprun Tram is its own instanced zone (similar to a dungeon or battleground), you cannot find Nessy nor the environment outside the glass tunnel anywhere else in the game world. The glass is impassible without exploiting, so there is no way to interact with Nessy other than targeting it.
Some people say thay got to Nessy by charging through the glass (as a warrior) but that is still in question, and even if you do you will most likely get banned.