I recall that it was a boy and a pen falls from the sky. I think he draws a fish and it comes to life and jumps into the river or pond. He takes it home and has a blue wall in his room and he draws a hippo, and it comes to life. I just recall that they lived in a fishing town and lived by the bay or something like that. It was a 80s or early 90's movie
Plot seems similar to Otto is a Rhino (1983).
Topper is an optimistic young boy with a vivid imagination. He is also in love with Cilla, but she thinks he's very crazy. One day Topper finds a pencil and draws a rhino that comes to live! This changes everything.