Actually, Pennywise The Dancing Clown appears in
cycles of 27 years, but the cycle is not exact. "It" can
masquerade itself to
appear as whoever or whatever "It" wants, but mostly "It" appears as
clown.
In the book, the group of children return in year 1985 to Derry (in Maine) to kill "It", as they promised to do so in year 1958, when they first met "It", and nearly killed "It". Pennywise The Dancing Clown's real name was Robert "Bob" Gray.