Ultivore

A gold, cybernetic, battle machine Insectovore turned giant. Count Dregon captured a stranded midget alien with the power to shrink and enlarge anything. Dregon then forced the alien to enlarge a monster that would destroy Dex and the entire city of Leawood. Dex was outmatched by Ultivore's height, but the alien evened things out by making Masked Rider into a giant as well, much to the dismay of Dregon. With a giant-sized Rider Kick, Dex destroyed the Ultivore and saved the alien.

NOTE: This monster was taken from Kamen Rider J rather than BLACK RX. It was also the main villain of the movie and was known as the Fog Mother.