My creature is the Ra'shalak (translated from an ancient, long forgotten tongue as 'The Demon Among Us'). It's a shape-shifter, and it's true form (if it has a single 'true' form) has never been observed by anyone who's lived to tell the tale. It prays very specifically on the young, particularly introverts and loners, who's separation from their peers makes them all the more vulnerable. It stalks them silently, sometimes for days at a time, until it has studied their movements and behavioral patterns in exquisite detail. When it strikes, it devours its prey whole, before then assuming their exact appearance and mannerisms, even absorbing their memories, in order to live the life of the child it has just killed.
During this process it feeds, not on the flesh of the child itself (such things are little more than redundant by-products, waste, to the Ra'shalak), but rather the essence of youth itself. As the body it has assimilated matures into adulthood, the Ra'shalak discards it, returning to its true form and going in search of new pray. The Ra'shalak is only ever as old as the body it assumes, meaning that, so long as it doesn't have to wait too long between 'feeds', it has a completely indefinite lifespan. It's elusive nature means few anecdotes are known for certain, but it is rumored that certain Ra'shalak have been praying on the same bloodline, child after child, generation after generation, for over a thousand years.