Some quotes from this article about Shelby:
“Toy experts concede there hasn’t exactly been a great clamoring for a toy clam, but being an F.O.F. (Friend of Furby) can’t hurt. More than 40 milion have been sold worldwide since Furby’s 1998 debut.” [sic]
-------
Why a clam?
“With Furby a certain height and a teardrop shape, I wanted to really contrast that shape,” says Hampton. “So I thought, not as tall and bigger around.”
Although he considered a snake, he rejected it. “Snakes don’t seem quite as friendly,” he concludes.
-------
“The chip-laden Shelby boasts 50 percent more memory than his Furby friend and can utter three times as many phrases. And the more you play with Shelby, the more words he masters.”
-------
Now we know why Slither-by never happened but Shelby did. The article doesn’t reference Fishby, but Dave did mention he didn’t want kids putting Fishby in the water.
Archived link