I found the masterball, and the actual legendary bunny.

[PotaDOH]

I can't seem to do anything with either of them though. Does the girl I gave the normal frobbit to come back and give me a new quest involving these?

[LuVenBen]

they don't do anything sadly.

[Spyder Z]

I didn't put these two things together in my head, so I'm both ashamed of myself for not thinking about it, and suddenly disappointed the legendary frobbit isn't used for Anything. ;P