Synonyms for Heterothermic (same or very similar meaning)
WordNet sense 1 (of animals except birds and mammals; having body temperature that varies with the environment):
ectothermic, poikilothermic, poikilothermousWordNet sense 2 (having cold blood (in animals whose body temperature is not internally regulated)):
cold-bloodedFrom the ODE community, based on WordNetadd/edit