Weren't these things around when dinosaurs may or may not have masturbated with their left or right hands?
Mammals appeared in the Mesozoic (about 66 million years ago), but the kind that would develop into capybaras (and guinea pigs!) probably weren't around until about 41 million years ago, with capys appearing about four million years ago.
I haven't seen any information on capy masturbation; they probably do not use either hand, as they do not have hands.