It was time.
Trevor had outgrown the safety of his master's clumsy love and protection. While he'll never forget Neville, it was time to move on.
Layla, the Giant Squid, and Trevor's new relationship was uncertain but strong. Trevor couldn't keep sneaking off worrying Layla and Neville both when he's away from the other.
And so, flipper to tentacle, they swam through the portal at the bottom of the Black Lake.
Only to end up way, way in the past; Founders of Hogwarts era.
Their daughter, a basilisk, ends up befriending Salazar Slytherin.
Centuries later Fawkes, with a heavy heart, slays her old friends' only child to protect a friend of his original owner.