From PinataIsland.info, the Viva Piñata wiki
Blocked species are one of the new forms of trouble that Professor Pester has caused around Pinata Island. If you glance around outside your garden, you may happen to notice that some pinatas are blocked by Ruffians or other Ruffian-arranged obstacles. These pinatas won't be able to visit your garden, until you manage to free (unblock) them.
To unblock a trapped species, you first have to meet its Appear requirements. The species will then make an appearance in a video showing its trapped predicament. Once its Appear requirements are met, you can now free it by meeting its Unblock requirements. More details and help can be found in the FAQ.
Species with Unblock requirements
Unblocking all these species will unlock the credits from the main menu of the game.