Poikilotherms, a.k.a. "cold-blooded" creatures, are those which have no means to stabilize their internal temperature. Consequently, their internal temperature tends to fluctuate according to the environment.
Compared to homeotherms (a.k.a. "warm-blooded" creatures), poikilotherms are usually more vulnerable to extreme temperatures, especially cold. However, they also spend significantly less energy and need to consume food less frequently.
On Earth, most animals are poikilotherms, with the main exception of birds and most mammals. There exist rare examples of cold-blooded mammals, though, such as the naked mole rat and the extinct goat Myotragus.
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