What is the rather unusual choice of national animal for Mauritius? Why was this animal chosen?

The extinct dodo is the national animal of Mauritius.

The dodo has become a symbol of national identity in Mauritius, a kind of synecdoche for the island and its relationship to its colonial past.

https://www.atlasobscura.com/articles/mauritius-and-the-dodo

A synecdoche is substituting a more inclusive term for a less inclusive one or vice versa.

Though the dodo may be now synonymous with a kind of cursed stupidity ("going the way of the dodo" is a cliché on Mauritius as much as it is elsewhere) it did not waddle dumbly into extinction. They were naïve, but not without reason; after all, they had never met a predator.

The Dutch explorers did not hunt the dodo to extinction. By all accounts, it was not particularly nice eating.

The real problem was less the humans than what they brought with them. Cats, rats, monkeys, pigs, and other animals the colonists imported by accident or design were likely the ones who killed the bird off by feasting on its eggs and competing with it for food and resources. At a time when species around the world are facing similar threats, the dodo remains a bracing metaphor for ecological degradation-just not the way that we think. As is often the case, the dodo died not primarily from overt human villainy-blood-thirsty sailors thwacking birds on the beach-but rather by the all too human failure to consider the secondary effects of our actions-stowaway cats and rats-until it is too late to reverse them.

The dodo has never been officially chosen as the national animal of Mauritius. Instead, it holds a significant place in the country's history and culture due to its unique status as an extinct species and its association with the island's past. It is part of the coat of arms, appears on several coins and is often featured in the country's cultural and tourism representations. But again: The dodo is not the official national animal of the Republic of Mauritius.

https://en.wikipedia.org/wiki/List_of_national_animals