Hotness is the feeling of being 'too hot' in Creatures. It is a common side effect of disease in the Creatures series.
In C1, hotness and coldness do not cancel each other out by default - only in some genetic breeds such as the Yin Yang Norns. It is therefore important to give the norns something which reduces hotness, not increases coldness when they are sick, such as the gentian herb. The kitchen fireplace (which creatures often call "hot") makes the kitchen quite stuffy.
In C3/DS, hotness is reduced by a creature having enough water in their system - or by the creature being both cold and hot at the same time. If the creature runs a high temperature, they can run out of water and suffocate, so it is important to treat fevers. The Magma Norns have a higher tolerance of hotness than other creatures.