Wet tail is severe, often-fatal diarrhoea in hamsters (frequently caused by proliferative ileitis), seen as a wet, soiled, matted rear end with lethargy and loss of appetite — a veterinary emergency.
It’s most common in young, stressed hamsters (after transport, overcrowding or a diet change) and progresses fast, so it needs urgent veterinary care, fluids and isolation from other hamsters.