Final answer:
Sodium (Na) is an element essential to animals for various biological functions, including electrolyte balance and nerve transmission, but it is not generally required by most plants.
Step-by-step explanation:
The element required by all (or almost all) animals but not required by most plants is sodium (Na). Sodium plays a critical role in animals for electrolyte balance, maintaining membrane charge, and ensuring proper nerve transmission. While most plants do not require sodium for their growth or development, it is often present in them due to the abundance of sodium in the environment. Animals, however, must obtain sodium through their diet to meet their biological needs.