This is a novel I read as a kid/teen in the 90s, I'm not sure if it was new at the time or not.
I believe it was set in a small town, possibly in England. There was a farm, where some kind of creature was evolving throughout the novel. One big reason I remember this book so strongly is that the book featured illustrations of the creature at different stages of its evolution throughout the book. I can't remember if it was an alien or a genetic experiment of some kind.
There was a romance between a boy and a girl, they were probably teens.
Every few chapters there would be a new illustration of this creature growing. These were the only illustrations in the book, it was not a graphic novel.
I vividly remember how the creature looked, it was pretty scary - it started looking like a simple organism, evolved into a long "eel" like form, and finally a larger beast. It reminded me of a prehistoric sea-dwelling creature of some kind.
I wish I could remember more specifics! I can't remember how the farm was involved exactly but I believe it was connected with this evolving creature in some way.