The term 'natural to a creature' can be summarized by the crossword solution 'INBRED, ' which is a six-letter word. INBRED refers to the process of breeding between closely related individuals, such as siblings or parent and offspring. This type of breeding is often done to maintain certain characteristics in a species, such as size or color. Inbreeding can also lead to genetic defects, as it increases the chances of recessive genes being expressed.