In the mid-sections credit for the Shazam movie,
A caterpillar is talking to Thaddeus Sivana when he is locked in his prison cell.
We see this same creature very early on in the movie,
When the younger Sivana first enters the original Shazam's cave.
What is this creature and its significance in the DC universe?
I haven't seen the picture you're talking about, but I'm going to take a shot in the dark and guess that your "caterpillar" is that famous Golden Age villain, Mister Mind. Quoting Wikipedia (with added links):
Mister Mind is a fictional character, a comic book supervillain created for Fawcett Comics, and now owned and published by DC Comics. Created by Otto Binder and C. C. Beck, Mister Mind first appeared as a voice only in Captain Marvel Adventures #22; his first true appearance was in Captain Marvel Adventures #26 (August 1943).
One of Captain Marvel's primary villains, Mister Mind is a two-inch alien worm of high intelligence. Mind usually carries out his villainous plans through an organization called the Monster Society of Evil, significant as one of the first supervillain teams in comics to contain villains that a superhero had fought previously; prior to this, supervillain teams were composed of villains created just for that storyline. The Monster Society made its debut in Captain Marvel Adventures #22, and the resulting "Monster Society of Evil" story arc continued for two years in Captain Marvel Adventures, ending with issue #46.
This is what Mister Mind looked like, taken from the last link.
