heterotherm (n.), heterothermal (adj.), heterothermic (adj.)

https://doi.org/10.1351/goldbook.14740
Animal that partially regulates body temperature, sometimes maintaining its body temperature at a certain level, e.g., when active, but at other times allowing it to fluctuate with the environment.
Source:
PAC, 2009, 81, 829. (Glossary of terms used in ecotoxicology (IUPAC Recommendations 2009)) on page 896 [Terms] [Paper]