edit: In the subject you mean "+400 MH/s mining ltc" I think.

With a hash rate that high I wonder why anyone would bother using a pool.

Hypothesis:

• It's a former Bitcoin GPU miner with more than 500 7970s.
• Someone figured out how to mine LTC effectively with an FPGA or ASIC.
• It's a botnet.

I'd guess botnet. Especially so since the hash rate fluctuates throughout the day.  It's only about 294MH/s now.