Isso poderia ser " Latitude Zero " (1969)?
O enredo envolve pessoas sendo resgatadas e trazidas para uma base em uma montanha em uma ilha por um submarino. Aqui está parte da sinopse da Wikipédia que descreve a criatura do tipo grifo e os transplantes do cérebro.
Latitude Zero is a paradise hidden fifteen miles below the surface at the intersection of the Equator and the International Date Line. Gold is plentiful there and is used for protective clothing by the island's newcomers, while those who arrived in the 19th century prefer to dress as they are accustomed, as no one ages or dies. Diamonds are very common and used only in industrial use.
Dr. Malic, however, wishes to destroy the paradise of Latitude Zero, and uses giant rats and anthropomorphic bats in addition to James Bond-style devices against the undersea kingdom. Worst of all, after a cruel experiment grafting the wings of an eagle to a lion, he removes the brain of the unsuccessful Kuroi and places her brain in the creature. This proves to be his undoing when, at a critical moment, Kuroi turns against Malic as he prepares to fire an energy cannon at the escaping visitors, causing their destruction.
Você pode ver o 'grifo' em 1:17 no trailer e a parte em que ele tira o cérebro da mulher para colocar no grifo em 1:38. Você também tem vislumbres de híbridos de animais / humanos e do vulcão.