For Harry's form a Thestral might be good. Sort of invisible. Mostly to his friends but not so to most of his enemies. Can fly and has a generally bad reputation. And not a snake.
For Daphne a small grass snake.
Where's Ron/Hermione? I don't really care for stories where Harry is Uber-Powerful. Any great power really should have a not insignificant downside to keep it interesting. And rather than having him turn into a super genius learning new tricks consider him finding artifact that grants him somekind of power (without him being aware of all that it does). Harry has always struck me as kind of dumb but powerful.