The things I consider for if a species is essential:
1. Is it common in zoos overall?
2. Is it often a popular sight for the major zoos?
3. Does it add to a region in a meaningful way?
4. Is the species iconic and well known?
5. Is the species unique compared to others in the game?
And even then, this is all subjective. I am always going to have a bias towards how common animals are in my regional zoos. I think there is nothing wrong with people having a personal bias, people will relate more to animals that they know of locally. But it is silly to try and claim anything is objective.
1. Is it common in zoos overall?
2. Is it often a popular sight for the major zoos?
3. Does it add to a region in a meaningful way?
4. Is the species iconic and well known?
5. Is the species unique compared to others in the game?
And even then, this is all subjective. I am always going to have a bias towards how common animals are in my regional zoos. I think there is nothing wrong with people having a personal bias, people will relate more to animals that they know of locally. But it is silly to try and claim anything is objective.