Animal with no predators

Solution 1:

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:

• ... apex species (i.e. those at the top of a food chain)...
  _{Understanding impacts of fisheries bycatch on marine megafauna}

• Mugger crocodile is a predatory apex species in the wetland ecosystem...
  _{POPULATION STATUS OF THE MUGGER IN CHITWAN NATIONAL PARK, NEPAL.}

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.

Solution 2:

One possibility would be 'species not subject to predation'.

(Note: 'unpredated' also comes to mind; I have not been able to find an authoritative dictionary-based source to back this up, but the word is definitely used. For example:

'Distance of predated and unpredated nests of Honey Buzzard to the next Goshawk nest at the two study sites (means ± standard errors).' Source: https://www.researchgate.net/figure/264409021_fig1_Figure-2-Distance-of-predated-and-unpredated-nests-of-Honey-Buzzard-to-the-next-Goshawk )

Solution 3:

Top of its food chain

EDIT: initially I used top of the but have changed answer to top of its to emphasise there may multiple food chains with various tops.

food chain
noun 1.(ecology) a sequence of organisms in an ecosystem in which each species is the food of the next member of the chain
— Collins English Dictionary - Complete & Unabridged 2012 Digital Edition

An organism is

an individual animal, plant, or single-celled life form.

So using the above definitions at the top of a food chain, you find organisms (which may or may not be predators themselves) without natural predators.

To better show the multiple food chain approach here is snippet from a BBC revision page.

A food web is a network of interconnected food chains. It shows the energy flow through part of an ecosystem. The diagram is an example

The example above contains lots of information. Here are three food chains from it:

• oak tree → squirrel → fox.
• oak tree → earthworm → wood mouse → fox.
• oak tree → earthworm → wood mouse → owl.