Yes. it is a rare event. The probability is very low, but it is possible. The Buddha clearly said that coming out of the apayas is a rare event, but it does happen.
As we have discussed before, at the cuti-patisandhi moment, many strong kamma vipaka from the past come to the mind, and one is selected based on the state of the mind at that moment. It is possible that an animal may have had a strong good kamma vipaka from the past.
– There is a sutta about a frog who was listening to a desana by the Buddha. The frog was crushed unknowingly by a person’s walking stick, and it was born instantaneously in a deva realm. This deva, called Manduka deva, then realized how he was born in the deva world, and came back to listen to the same desana which was still in progress. The frog of course could not understand the desana, but his state of the mind (due the soothing voice of the Buddha) enabled him to grasp a deva bhava, based on a strong past good kamma.
Another possibility in this particular case is that it is only mentioned about a monkey bhava from the past. It does not need to be the bhava just before the human bhava when the Arahanthood was attained. It is possible, for example, that there was a dvihetuka human bhava in between the monkey bhava and the last human bhava where the Arahanthood was realized.