'plains-dweller' originates from English, specifically the combination of 'plain' + 'dweller', where 'plain' ultimately comes from Latin 'planus' meaning 'flat' and 'dweller' is formed from 'dwell' + the agentive suffix '-er' meaning 'one who lives (in a place)'.

'plain' changed from Latin word 'planus' to Old French 'plain' and through Middle English (e.g. 'plein'/'plain') became the modern English 'plain'; 'dweller' developed in English by adding the agentive suffix '-er' to the verb 'dwell' (from Old English 'dwellan'), producing 'dweller' to mean 'one who dwells'.

Initially it referred simply to 'one who lives on a plain or open flat land'; over time it has retained that core meaning and is used for both humans and animals inhabiting plains.