@Tobias: Cuti-patisandhi means the kammic energy for the previous bhava had been exhausted (cuti) and grasping (or attaching to) a new bhava.
So, that Arahant WAS in an animal bhava (monkey bhava), and was born a monkey many times and exhausted that kammic energy. Then at the cuti-patisandhi moment, human bhava was grasped. So, he attained Arahanthood after being born a human.
The sequence is: Monkey bhava (where he was born a monkey many times). When the kammic energy for the monkey bhava expired, human bhava grasped at the cuti-patisandhi MOMENT. Then he was born a human within that human bhava. Then he attained the Arahanthood.