Sounds like a cross between The Blood Beast Terror (1968) and The Maze (1953), though Blood Beast Terror was woman turns into moth and The Maze was man becomes...(ahem) ...frog though it did have a scene where the creature is taken nightly from a locked room to a pond within the Maze. Actually The Maze is one of my favourite films and I found it quite creepy when I first saw it.
What's confusing is the taking the snake out to a pond? Not sure why you'd need to do this.