Slimes. Don't mess with 'em.
Slimes in hack/mine will appear in several elemental forms ranging from black to rainbow-colored. They are a good source of exp, particularly while spamming area of effect abilities, because they take out all of the miniature slimes spawned by larger ones.
A level 4 blue slime.
Most slimes are either the smallest size or second to that. Boss slimes are the larger size.
There are "epic" slimes, which spawn large slimes when killed (Confirmed). If one of these is spotted inside of a tower, its massive size spans from the floor to above the ceiling, making its name go into the room above and impossible to read. Sometimes it glitches through walls.
Two yellow slimes and their level is hidden and the big one has a very long name.