Vernon Wells. Zombie since 2011.
Vernon Wells has tried to come back from beyond the grave several times but always manages to get pulled back with the undead. After alternating brilliant seasons (2003: .909 OPS; 2006: .899 OPS) with disappointing ones (2007 : .706 OPS; 2009: .711 OPS) with several seasons bouncing in between, Vernon cemented his place among the damned in 2011. He's had a few good 2 weeks runs, but hasn't managed an OPS above .682 in the past 3 seasons.
Placido Polanco: Zombie Since 2012. Polanco has managed to put up some big seasons in his career, his 2005 (.331/.383/.447) and 2007 (.341/.388/.458) seasons especially come to mind. He's been around the league, bouncing between the Cards, Phillies, and Tigers before settling in Miami this year. Placido joined the ranks of reanimated brain eaters in a slow drop off since 2007. But I can only really fairly call him a zombie since 2012, when he finished the year with a wRC+ of 71 (league average is 100). He seems to have really shed any sort of human form this year and become a full fledged ghoul, bottoming out with a wRC+ of 57.
Yuniseky Betancourt. Zombie since forever. Betancourt has been nothing but a maligned walking corpse since his major league debut. In his very best of seasons 2006 and 2007, he was a slightly below average hitter and an average fielder (according to Fangraphs fielding tool). It's been been a black hole of terrible pretty much ever since, never eclipsing a .700 OPS since. He has even managed a .579 OPS so far in 2013.
Ichiro Suzuki. Zombie since 2011. Ichiro has been depressingly consistent since 2011, he seemed to get a bit of a spark after being dealt to the Yankees in 2012, however it didn't quite carry over to this year. He still holds some value as a player because his defense is still above average. However at the plate, he joined the living dead quite suddenly. His free swinging approach really hampers his value now that he can't slap any pitch for hit. His batting averages have remained somewhat respectable, but his inability to get on base in another manner (OBPs of .310, .307, and .307) and his total lack of power ( SLGs of .335, .390, and .346) have made him a one trick (zombie) pony not worth riding.