In the second half of "The Dark Knight Strikes Again" by Frank Miller, Brainiac shows up over Metropolis to fight Superman. Instead of his normal appearance, however, his ship/body looks like a giant toad. Then later we see him perched on the top of Luthor's shoulder, presumably in his real body, and it is the same frog-like appearance - only smaller.
As far as I know, Brainiac has always appeared in a humanoid form, with his ships often resembling a disembodied head with tentacles. Has Miller or anyone else associated with the comic discussed why this drastic change in Brainiac's physical appearance?