As far as I can tell, there isn't a real term for this. Apex predators sit at the top of the food chain and are not preyed upon. There are herbivores who are basically in the same position, such as elephants, rhinos and hippos (they're mostly herbivorous).
From what I can tell, these animals are:
- Large (Usually larger than the predators)
- Have a thick hide
- Tend to be social and move in groups
- Can use their size and other defenses to mess up any opponent
I know there is no such thing as an untouchable animal. Opportunistic predators may target the old, weak and young. At the same time, hyenas sometimes steal prey from lions.
So, what are the other common traits among these "apex" herbivores?