I’ve used riddles in my D&D adventures. I have, therefore, watched a group of intelligent people overthink a mind puzzle for much more time than you would expect, as I peruse the rest of the options from ‘riddles for kids‘, wondering if I could have been more generous.
I am pretty sure that most encounters with a sphinx would end in violence…
Sphinx

The Sphinx already present in D&D – the Gynosphinx and the Androsphinx – have a lot going on. All sorts of physical and magical abilities, and they are even more elaborate if they are in their lair. Fear effects, aging effects, spells… the Harry Potter Sphinx is much more run-of-the-mill. A Sphinx is hardly a mundane thing, but things in Fantastic Beasts and Where to Find Them tend to be a little simpler, more likely to live in the real world unnoticed. Monsters in the D&D world exist because…well, because…
So my conversion reflects that more tangible nature. It also leans into the description of their ferocity. So, whilst the Sphinx of D&D are previously powerful in a variety of ways, this Sphinx is a highly intelligence person-cat, that becomes a physical brute when it’s had enough of your wrong answers to its riddles (or your flippant refusal to take part, of course).
This alternative treasure guard lacks the magical prowess and weird magics of the original monster, but it is a more intelligent and cunning thing (which you should lean into heavily with your roleplay; I’m envisioning a heightened level of smugness) and is also a more dangerous combatant. Given that the Sphinx is mostly encountered in a tomb or vault, it’s impressive speed makes it a particular menace.
Just remember: your sphinx is a highly learned being, but the puzzles you issue to your players should be aimed at children. This is a well established fact about all D&D players, regardless of IQ levels. And your party won’t notice either. They will see your junior-level riddles as masterful works to test even the greatest minds.
Thank You For Reading
All the Fantastic Beasts monsters are being translated into Dungeons & Dragons, and you can find the stat blocks here!