(UPDATE 4/22/2013) It took workers 4 hours to remove the rats and make sure the transformer was OK. Cooling of Reactor 2 SFP resumed. The water temperature rose 0.1 degrees Celsius to 14 degrees Celsius during the halt.
They were discovered by a team of workers patrolling the premise. One of the rats, the gray one in the photo below, looks like he's been there quite a while.
From TEPCO's Photos and Videos Library, 4/22/2013:
The box that these rats were found is that small box on the ground on the left-hand side of the photo below:
Cooling of Reactor 2's Spent Fuel Pool was halted as of 11:36AM to remove the rats and make sure the transformer was not damaged. The temperature of the water in the SFP was 13.9 degrees Celsius. TEPCO estimates a few hours for the work of removing the rats and ensure safety of the transformer.