The Blob Monster is a one-time villain from the Cartoon Network animated series, The Powerpuff Girls, only appearing in the episode "Catastrophe".
In "Catastrophe", The Blob Monster terrorized Townsville, by tearing down buildings and smashing things. The Powerpuff Girls flew right into action to stop him. First, they politely asked him to stop destroying their town, but before Blossom could even finish talking, he responded by socking them all, with one punch, and sending them flying into buildings across town. Blossom asked herself, why these monsters always want to go about these fights "the hard way", and she shot right out to get physical.
Bubbles went flying right into The Blob Monster, and went to kick him in the head, but because of the slimy material he was made out of, Blossom, just went right through him, not harming him at all. She came out the other side of The Blob Monster, completely covered in slime. Blossom tried to do the same thing, but to the same effect. Buttercup tried doing the exact same thing her sisters did, wholeheartedly expecting different results, and her sisters cheered her on, but to her surprise, she just went right through The Blob Monster and came out the other end, covered from head to toe in slime.
Blossom tried using her ice breath on him, and managed to freeze him solid. She thought she had won, when he was immobilized. The Powerpuff girls flew away, triumphantly, while the citizens of Townsville cheered for them. However, The Blob Monster broke free from the ice and went right back to destroying buildings again.
The Powerpuff Girls all tried using heat vision on The Blob Monster, by bombarding him with their laser eyes. This caused The Blob Monster's slime to boil, making him explode all over Townsville. Bits and pieces of The Blob Monster were scattered all over the buildings and people and ground, and The Powerpuff Girls, once again thought that they had won. However, all the bits of slime started to move on their own and gather together in the middle of the road. The Blob Monster regenerated from this and went back to destroying the town again.
The Powerpuff Girls' last resort was to form Furious Fiery Feline. When they did this, The Blob Monster gave the surprising reaction of grabbing the Furious Fiery Feline out of the air and hugging it. He yelled out "Kitty!" and then told Furious Fiery Feline that he'd been looking all over for her. Surprised, The Powerpuff Girls turned back to their normal state, revealing to The Blob Monster, that they were not his kitty. The Blob Monster was angered by this, and so he slapped The Powerpuff Girls out of the way and screamed "Where's my kitty!?"
The Powerpuff Girls realized that this monster wasn't really much of a bad guy, after all, since all he was trying to do, was find his lost cat. They took it upon themselves to help him, instead of hinder him, since this would not only be a more effective way, but also a more ethical way to stop him from destroying Townsville.
The Powerpuff Girls searched The City of Townsville, looking for a cat, and they found a stray, walking through the alley. The Blob Monster discovered this and was happy to have it at first, but then he realized that this cat was not his cat. The Blob Monster got angry and threw the cat through the air. The Powerpuff Girls had to chase after it and save it, before it crashed into a building.
Blossom tried asking The Blob Monster questions about his cat, inquiring what it looks like, and how to find him, but he didn't listen to them, and just continued ransacking the town. Bubbles used her animal calling powers and her supersonic scream, to reach out to all the cats in Townsville and lead them over to where she was, in hopes that at least one of them would be the cat The Blob Monster was looking for. The Blob Monster scrounged through all the cats, but not one of them was his. Once again, he got angry and threw all the cats into the air. The cats rained down on Townsville and The Powerpuff Girls had to scurry all over the place, catching the cats, before any of them hit the ground.
Finally, Blossom spotted a pussy cat, wedged between The Blob Monster's butt cheeks. She yanked him out of there and presented him to The Blob Monster. At long last, The Blob Monster was satisfied, as he had finally found his cat. The Blob Monster happily reconciled with his cat and pet his head. The Blob Monster told his pet cat, that they should be heading home now, and he happily took off with his cat, to go back to Monster Isle.