The Grim Adventures of Billy & Mandy

The Crawling Niceness (2x15)

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Billy takes a big egg found in Grim's trunk and hatches it expecting a duck. Instead, Jeff the Spider is born, who considers Billy his father. Despite Jeff trying to please Billy in any possible way, Billy is terrified of Jeff and wants him squished. In the end, we see that the exterminator didn't squish Jeff because he is a nice bug.