The collar is a heirloom of the Doo family.
A Pup Named Scooby-Doo
Scooby's parents gave him the collar as a present, which he didn't like it, but was still forced to wear it. However, it became the target of the Ghost of Buster McMuttMauler, an enemy of the Doo family, claiming the collar was his. The gang used it to lure the ghost into a trap, but it backfired, and the ghost obtained it. But thanks to Scooby, he got trapped in tire swings. The ghost was unmasked as Mr. Trixenstuff, the trainer, who wanted to sell the collar because it's worth a lot of money (even if it was ugly). The next day, Scooby still didn't like wearing the collar, though Mama-Doo confessed that Daddy-Doo always hated to wear it too. And since it's her son's birthday, she allowed him to take it off, pleasing him.