Final answer:
An animal that can stay warm in cold weather due to controlling its own body temperature is an endotherm. Endotherms generate internal heat to keep their body temperature constant, which allows them to stay active in varying environmental conditions.
Step-by-step explanation:
An animal that can stay warm in cold weather because it can control its own body temperature is known as an endotherm. Endotherms maintain a constant body temperature regardless of environmental changes, thanks to the heat produced internally, which enables their cellular processes to function optimally even in cold environments. This is in contrast to ectotherms, which depend on external sources to regulate their body temperature and often exhibit a body temperature corresponding to that of their environment.
Regarding the specific question on which type of animal maintains a constant internal body temperature, the answer is endotherm. Unlike endotherms, poikilotherms have constantly varying internal temperatures, and heterotherms are capable of switching between endothermy and ectothermy.