John Gill Commentary Luke 9:19

"And they answering said, John the Baptist; but others [say], Elijah; and others, that one of the old prophets is risen again." — Luke 9:19 (ASV)

They answering said, John the Baptist
This was the opinion of some who thought that he was risen from the dead, as in (Luke 9:7) .

but some say Elias ;
the prophet, and the Tishbite; who according to the Jewish notion, was to be the forerunner of the Messiah, so in (Luke 7:8) .

and others say: that one of the old prophets is risen again ;
thus were they divided in their sentiments about him. (See Gill on Luke 9:8)