From the Super Mario Wiki
Net Bombs are a type of Bob-omb seen in The Super Mario Bros. Super Show! episode "Two Plumbers and a Baby".
Net Bombs resemble Bob-ombs in appearance, but instead of exploding like regular Bob-ombs, Net Bombs instead transform into nets, which can be used to capture beings. When changing into a net, a Net Bomb will simply pop and be surrounded by some smoke. In The Super Mario Bros. Super Show! episode "Two Plumbers and a Baby", Net Bombs were utilized by King Goo Goo Ga Ga Koopa's Albatoss minions, who proved to be somewhat incompetent in the use of Net Bombs.