In the episode script, Iron Rocks is said to have been specified as a human-shaped robot made of the debris from sunken Battleship Yamato as it was to come ashore heading for the TDF Far Eastern Base with the costume to require two actors like Dodongo.
The trailer of the time also seems to have announced Iron Rocks to come ashore in the episode to come next.
While Alien Mimy is known as an alien who didn’t show their true identity except the appearance of their characteristic spacecraft in the show, it is unlikely that Narita drew any design of Alien Mimy.
I assume the starfish-shaped spacecraft should have been designed by Noriyoshi Ikeya who was to take over kaiju designing from Narita shortly afterwards.
The Ultraseven Pictorial has an article of the time about Iron Rocks coming ashore with four legs along with the illustrated Alien Mimy while describing the starfish spacecraft as their undersea base.
As the four-footed Iron Rocks and Alien Mimy seem to have appeared in the manga version of Ultraseven drawn by Jiro Kuwata, the alien illustrated in this article might have been based on the one drawn by Kuwata (I realize the alien has a kind of Kuwata feel to it vaguely).
The plot of an aliens’ undersea base for invasion with a battleship robot seems to originate from one of the episodes of the tokusatsu show “WoO” left unproduced that was being planned along with “UNBALANCE” while the latter resulted in “Ultra Q” finally.
The same plot also came close to being realized as one of Ultraman episodes that was left unproduced either featuring a kaiju called Yamaton shaped like Battleship Yamato.
Yamaton seems to have appeared in the manga version of Ultraman drawn by Daiji Kazumine with the design worked out by Tohl Narita. It is surprising to learn this story went through such a long history over the three products (WoO, Ultraman and Ultraseven) until it got realized.
Tohl Narita: (about Yamaton) “I drew this design for a manga after the show Ultraman ended.”