YA novel - kid finds some kind of time traveling creature

I have a very, very vague recollection of this book that I read around 1999-2001.

A kid or tweenager finds some kind of creature -- the visual image in my mind is of something like a horseshoe crab but that's hardly reliable -- that I think can time travel, or allows him to time travel. Unfortunately that's about all I remember of it, but it was such a distinctively weird concept that hopefully someone else remembers it as well.

The overall vibe seemed "dark" to me, as a young person. There might have been another kid involved later on.