"Mr. Bonkers" was the cat owned by Donna, only seen in the episode, "Kiss of Death". In the episode, Eric runs over the cat by accident after Red told him to put the Vista Cruiser in the garage. Donna is furious, but she forgives Eric later.
Mr Bonkers is also referenced in the episode 'Donna's Story' in which Donna vents all of her frustrations about her relationship with Eric into a story set in the nineteenth century, one snipet of dialouge as follows:
Donna: But alas, I must give back this promise ball-and-chain, I wish to spend my days travelling the world with my trusty cat named Sir Bonkers.
Eric: Well what if I told you that I rean over the cat named Sir Bonkers and the cat named Sir Bonkers is no more.
Donna: You monster!
Eric: And from this day forth, I will hunt down and kill every cat named Sir Bonkers... Or at least... Spray them with water, which everyone knows they hate.