The plural of "mongoose" is "mongooses".
And for all the baseball fans, it's not RBI's, nor RBIs; it's simply RBI.

And the only (Oh crap! I was doing so well until I started this sentence with a conjunction.) major league announcer who uses RBI correctly is Jon Miller.

For the non-baseball literate, RBI is the abbreviation for Runs Batted In.

Seamus