It was a scruffy-looking male dog, with white fur.
When caught, it snarled.
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.