Bender's guinea pig was Bender's former pet. It died prior to the V-GINY rampage of 3010 (6ACV02). Likely causes of death include the numerous crashes of the Planet Express ship, the rebirthing process, or Bender's explosion after being swallowed by the Cyclophage (6ACV01). The Guinea pig could also have just died of old age, as it might not have been in Bender's chest cavity. Since there has been only one death of a guinea pig on screen in canon, it is possible that Bender's guinea pig is the one Nibbler ate in Bender's Big Score. His lack of reaction at the time would be understandable due to the influence of the effects of the obedience virus. Bender is noticeably saddened by remembering its death.

Additional Info

Quotes

Fry: Did anyone else feel aroused and jealous and worried?

Bender: I haven't felt much of anything since my guinea pig died.