Listen to all of Chico's audio briefings. This is done by selecting a mission, choosing "Briefing Files" then entering the Chico file library. Listen to all the tapes. Then select Extra Ops and select "[029] Item Capture". Start the mission, take care of the three guards, and destroy the item crate. Collect its items but instead of going to the extraction point move south to the beach at Playa del Alba. When you enter the beach, take a left and go down the dock. Miller will notify you about a strange animal at this location. Run down the dock to find Trenya, from the Monster Hunter series. After the dialog you can now access "[121] Hunting Quest: Rathalos" in Extra Ops. Complete it to unlock other Monster Hunter related missions.