You are correct that the eels were not named in ST II. However, they were named in the script.
Outside of the script, they are also named in Vonda M. McIntyre’s 1982 novelisation of the movie, Shane Johnson’s (as she then was) book Star Trek: The Worlds of the Federation (1989) and in Greg Cox’s novel To Reign in Hell: The Exile of Khan Noonien Singh (2005).
You are correct that the eels were not named in ST II. However, they were named in the script.
Outside of the script, they are also named in Vonda M. McIntyre’s 1982 novelisation of the movie, Shane Johnson’s (as she then was) book Star Trek: The Worlds of the Federation (1989) and in Greg Cox’s novel To Reign in Hell: The Exile of Khan Noonien Singh (2005).