Before turning to pea plants, Mendel attempted to breed mice.
The familiar account says that he kept mice in his monastery quarters and was interested in tracking inherited differences such as coat color, reportedly including crosses between differently colored varieties. His bishop objected to the work because breeding animals involved sexual reproduction and ordered him to stop. Mendel then pursued controlled crosses in the monastery garden, where pea plants became a far more productive research subject.
Peas offered several practical advantages: they were easy to grow, many varieties bred true, and their flowers could normally self-pollinate while still allowing Mendel to control pollination by hand. Their sharply contrasting traits also made results easier to count than complex animal characteristics.
The mouse episode is widely repeated in biographies and histories of genetics, but the surviving documentation is limited. Later colleagues remembered that Mendel had bred mice, while Mendel left no detailed record of the experiments. That uncertainty affects the story’s evidence, not the canonical trivia answer: mice.