Mus macleari [now Rattus macleari]- Maclear's Rat
This extinct indigenous rat of Christmas Island is thought to have been the primary population control for the local crab species, along with the also-extinct bulldog rat. Between those two rodents, and the local Christmas Island shrew (not sighted since 1908 and presumed extinct), the Christmas Island red crabs that provide a somewhat-unnerving migration spectacle, were kept at a level thought to be about one-half what they were at their height. These days, the aptly-named "yellow crazy ant" that was inadvertently introduced from Australia, has cut the red crab population by a third, but unlike Maclear's and the bulldog rat, the yellow crazy ant has no population control of its own, and may one day entirely wipe out the red crabs.
The Maclear's rat is thought to have gone extinct both due to humans killing them, and the introduction of black rats to the island, when the Challenger expedition landed there in 1876. The black rats carried a trypanosome which affected them to a mild degree, but would have wiped out any non-acclimated species that acquired it in large numbers.
Proceedings of the Scientific Meetings of the Zoological Society of London. 1887.