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It was a scruffy-looking male dog, with white fur.
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Skills and abilities
Only while disguised as Scooby-Dee, did it display perfect English, imitating her voice and southern accent.
The Scooby-Doo Show
The impostor replaced Scooby-Dee, when she was traveling on a train with the gang. Dee left her berth to get a drink of water, and the Phantom of Milo Booth grabbed her, muzzled her, and put her into a sack. The impostor took her place so that the gang didn't know she was dognapped. The impostor gave himself away when Scooby-Doo and Scooby-Dum wanted a kiss. He was unwilling to kiss them, so he pulled back their lips and sprinkled them with water instead. This caused them to get suspicious since the kisses didn't feel right.
In the end, the impostor was unmasked, and the plot was revealed: he was going to make money for his master, Jim Moss, by making new Scooby-Dee movies in her place.