Bumbercatch: No, they are not. Oakley, the Puple Soul, is the only one.
"WHAT ARE WE GOING TO DO NOW, MOTHER?" Papyrus asked as he and Sans tagged behind the human, leaving Snowdin as they headed into Waterfall.
"I'm going to show you the rest of the Underground." Casey answered as she walked, glancing back at the skeleton. He was now taller than she was, with Sans similar to her in height.
"are we goin' t' the wishing room?"
"Yes, we are. And we're going to go beyond that." Casey replied. When they got to the bridge puzzles, Papyrus helped her with solving them. The trio walked into the Wishing Room, hearing the rushing water fade away.
"WOWIE!" Papyrus says as he looks around, amazed. "THESE ARE VERY ODD FLOWERS!"
"Those are Echo Flowers. They repeat the last thing they heard over and over again. If you're really quiet when you walk up to one, you can hear it."
Papyrus glanced at Casey, then carefully walled up to one of the flowers and lowered his skull to try and listen. Hearing the whispering, he jumped back. "WOWIE! YOU WERE RIGHT!"
Casey smiled, then headed further into the room. Sans and Papyrus followed her. "Some of Waterfall is a museum, with plaques on the wall detailing the war between monsters and humans."
"Cool."
"WHY WAS THERE A WAR?" Papyrus asked.
Casey went silent for a moment, folding her hands. "... I think it was because they were scared." She finally says as they start down the long hall, the glowing plaques lighting their way.
"OF WHAT?"
"Monsters have the ability to take a human's Soul, even though it requires a lot of magic. Humans were scared of this power..."
"WHY DIDN'T THE HUMANS TELL THE MONSTERS THIS? WOULDN'T THIS HAVE AVOIDED THE WAR?"
Casey glanced at Papyrus. "That makes a lot of sense, doesn't it?... But people can do foolish things when they're scared."
"AS FOOLISH AS STARTING A WAR?"
Casey smiled a little. "Yes. As foolish as starting a war."
The group continued walking through the Underground for a few hours, finally turning around before they reached the Hotlands. Thankfully, Undyne didn't seem to be around anywhere. On the way home, they found the small sprout of an Echo Plant. After getting badgered by Papyrus, Casey carefully dug it up and they carefully carried it Home.