There was a pet cat that Ray and Robert had as kids. At some point in their childhood, the cat was either harmed are killed due to Robert's size and strength. He either hugged it to death or pet it too hard. It's not stated what happened exactly, it's only mentioned in the episode "I Love You" in Season 1 of the series.
It's unknown whether this is the same cat "Whiskers" that's talked about in the episode "Why Are We Here?".