What is the name for the type of energy formed in fossil fuels?

Fossil fuels are stores of chemical energy. The chemicals are various hydrocarbons.

The original source of the energy locked in these chemicals is Sunlight. The fuel formed from the remains of plankton that were buried beneath the Earth millions of years ago. Geothermal processes converted the remains of living creatures to the hydrocarbons that we now call fossil fuels. Fossils do not make energy. Fossils are usually from larger animals and plants, but the term fossil fuel is one of common usage.