A Mouse Goes Toe To Toe With John Cena

There's a new challenger for the WWE belt ... a ferocious .... teensy-weensy mouse!

It all went down last night at the Valley View Casino Center in San Diego on Monday Night RAW -- John Cena stormed the ring to attack WWE champion Alberto Del Rio ... and the two were photo-bombed by a tiny mouse.

Thankfully, no injuries were reported.