The spelling of the word "rat chinchilla" follows the conventional English orthography rules. The first word, "rat," is spelled as it sounds with the IPA transcription of /rat/. The second word, "chinchilla," is spelled with the phonetic transcription of /tʃɪnˈtʃɪlə/. The "ch" sound in "chinchilla" represents the voiceless postalveolar affricate /tʃ/, while the "il" sound is pronounced as a short /ɪ/ followed by a lateral approximant /l/. Overall, the spelling of "rat chinchilla" accurately represents its pronunciation.