Inter-Dimensional Monster

This horrid Giant Monster was summoned by Dexter from another dimension using his untested Inter-Dimensional Doorway. It was the very first destructive force (aside from Dee Dee) that Dexter ever fought and did not win against.

Appearance
It is a giant gelatinous pink monster with numerous tentacles and many yellow eyes with eyelashes and large red lips with sharp teeth.

Deedeemensional
When Dexter was finally fed up with Dee Dee's constant idiocy, he used his untested Inter-Dimensional Doorway to transport her to another dimension, but instead this monster came from the other side of the doorway and wrecked havoc on Dexter's lab. Despite Dexter's best efforts, he was unable to defeat it and the Monster swallowed Dexter whole. With his last remaining moments Dexter wrote a note to himself saying not to turn on the doorway which he gave to Dee Dee so that she would give it to himself in the past using his Time Machine. Upon arriving in the past, Future Dee Dee was unable to give Past Dexter the note due to his intolerance towards Dee Dee's presence. Eventually he finds out about Future Dee Dee upon seeing her with her past counterpart and tries his best to earn her favor and get the note, but unfortunately earning their good favor proved most troublesome, causing Dexter to lose his patience and open the Inter-Dimensional Doorway as he had done before, causing the events to repeat themselves with Dexter getting swallowed again by the Monster and writing a note. As he hands the two Dee Dees his note, Future Dee Dee gives him her note which Dexter realizes is the note he just wrote, causing him to figure out that the Dee Dees will always fail in their mission. He tries to stop the Dee Dees from repeating themselves but they disappear into the time machine, leaving Dexter to his doom once more as the Monster happily digests him.

Despite this, later episodes show Dexter and Dee Dee alive and well, implying that the Dee Dees did succeed in saving Dexter... eventually.

Sports A Poppin
After Dexter endured a long day of sports games with Dad, Dee Dee let loose the giant monster once again which broke out of the lab (and even the house) and began to attack Dexter on the front yard. This time however, Dexter proved to be far more capable and prepared to handle the giant brute, easily fighting against it equally by using the sports equipment his father gave him along with a laser gun.

Trivia

 * The Inter-Dimensional Monster made frequent appearances in promotional material and other Dexter's Laboratory media during the 90s where it was usually shown fighting Dexter.