From Latinpolypus, from Ancient Greekπολύπους(polúpous). The alternative form, pulpo, is attested in a few medieval documents from northern Galicia and denotes either the earlier loss of post-stressed vowels in the northern regions, before the lenition of intervocalic /p/, or the early expansion of an Asturian form.
1289, Francisco Javier Pérez Rodríguez, editor, Os documentos do tombo de Toxos Outos, Santiago de Compostela: Consello da Cultura Galega, page 698:
duzeas de puluos
dozens of octopuses
1417, Ángel Rodríguez González (ed.), Libro do Concello de Santiago (1416-1422). Santiago de Compostela: Consello da Cultura Galega, page 75:
Iten que se venda o pescado en esta maneira: a libra dos sacadores et das sollas et dos bodiõos, dos polvos et das fanequas et das rayas et das langostas et das sibias et das luras a tres dineiros cada libra
Item, they should sell the captured fish in this way: the pound of sacadores [?], of plaices, of Baillon's wrasses, of octopuses, of poutings, of stingrays, of lobsters, of cuttlefish and of squids, three diñeiros each pound
1495, Enrique Cal Pardo, editor, Monasterio de San Salvador de Pedroso en tierras de Trasancos. Colección documental, A Coruña: Deputación Provincial, page 305:
abedes de dar et pagar a nos et a ho dito noso monesterio et a nosos soçesores huna liaça de pulpus, mays quatro pescadas por día de Natal, en quada hun anno
you should give and pay to us and to our monastery and to our successors, a liaza [=eighteen dry tied octopuses] of octopuses and four hakes by Christmas day, each year
“polbo” in Xavier Varela Barreiro & Xavier Gómez Guinovart: Corpus Xelmírez - Corpus lingüístico da Galicia medieval. SLI / Grupo TALG / ILG, 2006-2016.