"Estral" is a lesser known word that refers to the heat period of female mammals, especially horses. Synonyms for "estral" include "oestrous," "heat," "fertility," "reproductive," and "mating." These words are commonly used in veterinary and animal breeding contexts. In addition, other words that can be used to describe this period include "rut," "breeding season," "fertile phase," and "estrus." It's important to note that these words may also have different meanings in different contexts. However, when it comes to describing the heat period of mammals, these synonyms for "estral" can be useful in conveying the necessary meaning.