The Cat Creature is the Cat God himself
Going along with the Alternate Character Interpretation I think he's just some kind of evil cat entity that has been angered at Scooby ever since he caused the end of his sacrifices all those years ago. He came out when he saw the Scooby gang return and aimed to take his revenge. He got thwarted by not knowing there were other things afoot on the island. He also didn't like the fake palm trees of the loosely based on a true story movie much either. Velma's unsolved issue explains her "What's A Scrappy?" Line
Since we now know that Velma obsesses over proving past "real" cases to be unsolved hoaxes it stands to reason Scrappy a character present for a lot of them has also been selectively edited out of her memory. No one comments on it because they know she'll have a breakdown if they mention him. The real Velma is being held prisoner by a crazed fanclub
Before Curse of the 13th Ghost took place, Velma met one of her fan clubs. Happy that she has achieved some notice with people, she spends the day with them. Unfortunately, she does not realize that these fans were upset with her because she easily embraces the supernatural and, in their eyes, ruins their shared image of her as a young woman deeply rooted in science. The fans led her to an abandoned cabin in the woods, claiming it was their secret clubhouse. Once inside, they attacked, tied her up, and restrained her. The club’s leader, who orchestrated the whole thing, changed her appearance and took on Velma’s identity, with Mystery Inc almost none the wiser. To this day, the fanatical imposter remains, obsessively maintaining her and the club’s perceived image of Velma while her co-fanatics keep the real Velma hidden away at the cabin. The sheriff will be revealed as a monster in a future movie
Why do you think the Sheriff wants the gang to quit so badly? He is planning something big and he doesn't want those meddling kids to interfere.- Confirmed sort of, in "Happy Halloween Scooby-Doo" he turns out to be the main villain and it turns out he's actually just impersonating the sheriff.
The fourth cat creature really was just someone in a mask working with the other three and they just lied to protect their friend due to Evil Virtues and/or to avoid additional jail time for the life-threatening damage that fourth one had inflicted
Velma is Dracula in Disguise.
Explain how else she had the werewolf's postcard. Only Dracula, his monsters. She is trying toUphold The Masquerade