Old Persian
Median is "presumably" a substrate of Old Persian. The Median element is readily identifiable because it did not share in the developments that were particular to Old Persian. Median forms "are found only in personal or geographical names... and some are typically from religious vocabulary and so could in principle also be influenced by Avestan... Sometimes, both Median and Old Persian forms are found, which gave Old Persian a somewhat confusing and inconsistent look: 'horse,' for instance, is [attested in Old Persian as] both "asa" (OPers.) and "aspa" (Med.)."