An update...

Alas! The mystery of the 2nd shrimp is solved. When my brother was cleaning his fish tank, he found the shrimp in the filter... and surprisingly.. It's still alive!!!! It's still gross looking.
The brown mouse has been returned ot PETsMart. It ruffed up my white mouse so now its fur is all messed up. That brown mouse was vicious. It took me several minutes to catch to put it in the carrier box because it scurried off too quickly. My last resort was grabbing it by the tail, and it bit me. My index finger bled a puddle of mud.
No wolf-cloud-eating-bunnies today, thank goodness.