The Count's Prince Pumpkaboo, despite being smaller than Jessie's Pumpkaboo, fell in love with her. Since Pumpkaboo evolved into Gourgeist, Prince Pumpkaboo stopped loving her.
Count Pumpka traded his Mawile for Jessie's Pumpkaboo, who evolved, due to trade evolution method, but gave Gourgeist back.
The Count traded his Mawile for Jessie's Pumpkaboo, but the trade was reversed.