Tobiah mocked that the wall would break down if what animal climbed on it?

Answer: A fox

Tobiah sneered, "If even a fox were to climb up on what they are building, it would break down their wall of stones!"

“Then Tobiah the Ammonite, who was beside him, said, “If even a fox were to climb up on what they are building, it would break down their wall of stones!””— Nehemiah 4:3 (BSB)