"Owning a invisible hippogriff or something like that." Because that would be a real challenge. But it seemed like they were moving onto more interesting creatures.

Interesting = Dangerous to Nigel.

The Prefect would pay money to witness that scenario of one mistaking a Hippogriff for a Griffin in your own backyard. Could be a horror show or awesome. Depends on how you view things.

"Griffins can be used as Guards"

Though Nigel guessed if you train the Hippogriff. It technically could do that. But he wont say that thought out loud.