The Epidii (Greek: Επίδιοι) were a people of ancient Britain, known from a mention of them by the geographer Ptolemy c. 150. Epidion has been identified as the island of Islay in modern Argyll. Ptolemy does not list a town for the Epidii, but the Ravenna Cosmography (RC 108.4) mentions Rauatonium, which is assumed to be Southend.
The name Epidii appears to include the P-Celtic root epos, meaning "horse" (c.f. Welsh ebol, “a foal”). The Q-Celtic equivalent would be *ekwos, which became Old Gaelic ech. It is suggested that they were named after a horse god, whose name could be reconstructed as *Epidios. The Q-Celtic equivalent would be *Ekwidios, which may be the origin of the Old Gaelic name Eochaid. The root *ep-anto-s, 'those who belong to the horse' or 'those who own horses', has also been proposed.
Although their name appears to be Brittonic/P-Celtic, some authorities suggest they were Goidelic/Q-Celtic speakers. Dr Ewan Campbell says "Ptolemy's source for his Scottish names was probably from the Scottish Central Lowlands, and may have transmitted the Brittonic form of a Goidelic tribal name, or even the external name given to the tribe by Brittonic speakers". Their presumed territory later became the heartland of the Goidelic kingdom of Dál Riata. Alex Woolf suggests that the Epidii became the Dál Riata, but argues that they were Brittonic-speaking in Ptolemy's time. He also suggests that the Hebrides, called the Ebudae by Ptolemy, were named after the Epidii.
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- The circumnavigation of Scotland compares Ptolemy and the Ravenna Cosmography for the south west coast of Scotland