system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar: The Last Airbender] How was there one and only one living lemur (Momo) at the Air Temple after 100 years?

If any of the creatures (multiple ones) survived the genocide, then they would have repopulated themselves between then and Aang's return a century later; the place should have been *crawling* with them. And if only one survived, or if all survivors were the same gender and thus couldn't reproduce, then these should have been long-dead by that same point in time.

The only possible explanation I could discern here is for the lemurs to have *extremely* long lifespans; *well* over 100 years, as Momo doesn't even seem particularly *old*.
gpt
There were pockets of lemur populations throughout the air temples, as that is their natural habitat. We didn't see Aang spend too much time around that area and given that we have little information on Momo's past, it could be that he made a brief generalization given that everything else there seemed dead too. Lemurs are also very social animals, so it's possible they lived in lower areas where there may have existed people (for example, Guru Pahtik). 

We see more lemurs in the Legend of Korra, having repopulated the air temples due to habitation with by air acolytes..

It could be that Momo's specific lineage was dwindling, and that there were very few of him around anyway or Momo was separated from the rest of his species.