The term apex species is fitting. This term is not widespread, but, being obviously related to "apex predator", it will be easily understood. The term is used here, for example.
After looking further into this, I found some evidence that the term is used research papers:
You might also be interested in the term megafauna, which includes megaherbivores. According to Wikipedia:
Megafauna – in the sense of the largest mammals and birds – are generally K-strategists, with high longevity, slow population growth rates, low mortality rates, and (at least for the largest) few or no natural predators capable of killing adults.
Megafauna includes a number of species that aren't very large, like white-tailed deer and humans.