I have visited Tonga about half a dozen times with several expedition companies.
Being small and remote islands means there is not much land-based wildlife. The only mammals are flying fox.
In terms of wildlife watching, Tonga is best known for it's Humpback Whales and the chance to swim with them.
The capital is Nuku'alofa and on the island of Tongatapu. On the other side of the island are an impressive line of blowholes.
I have visited the volcanic island of Tofua three times but not landed.