The stalker, E, is a monster in NetHack. They are always generated invisible, and are therefore referred to as invisible stalkers. Eating stalker meat, whether from the corpse or from a tin, will either turn the player temporarily invisible for 50-149 turns (if they are not already invisible) or permanently invisible with the see invisible intrinsic (if they are already invisible, temporarily or otherwise). When wearing a Mummy wrapping you are not invisible and therefore are not eligible to receive the see invisible intrinsic or the permanent invisibility intrinsic. Killing and eating two stalkers in rapid succession is one of the most reliable ways of obtaining these intrinsics. It should be noted that eating stalker meat in the heat of battle is not generally recommended as it will stun you upon consumption.
The stalker is based on a Dungeons & Dragons monster known formally as the invisible stalker. As most versions of Hack and NetHack have automatically prefixed monster names with "invisible" when actions that refer to an invisible monster have been involved, using the full name would result in the egregious "invisible invisible stalker"; therefore the prefix "invisible" was dropped.
Nevertheless, for a very long time the symbol I was used to represent the stalker (I meaning "invisible"). This changed in NetHack 3.3.0, when the I symbol was appropriated to indicate the last known position of any invisible monster. The use of the symbol for elementals reflects the fact that they are really a species of air elemental.