The Water Horse

Wikipedia: "an adaptation of Dick King-Smith's children's novel The Water Horse. It stars Alex Etel as a young boy who discovers a mysterious egg and cares for what hatches out of it: a 'water horse' (loosely based on the Celtic water horse) which later becomes the fabled Loch Ness Monster. Emily Watson, Ben Chaplin and David Morrissey also star."

Jacket: "When a lonely young boy named Angus discovers a large, mysterious egg along the shores of Loch Ness, no one is prepared for what lies within. He soon discovers that the strange, mischievous hatchling inside is none other than The Water Horse, the loch's most mysterious and fabled creature! But with the Water Horse growing ten times its size every day, Angus finds it increasingly difficult to keep his new friend a secret."

Think of Free Willy in the Loch Ness. This is a family film with a wee bit of fantasy and magic.