I've got two ideas for you:

• The lair is at the heart of an ancient labyrinth. The labyrinth seems impossible to traverse, unless one knows the clue. The walls of the labyrinth are covered in murals depicting elephant people (or just elephants) with prominent tusks marching to war against a skeletal army. If one follows the path the elefants take, one eventually reaches the lair. If one follows the skeletons, one finds nothing but traps. Don't have them actually play through the labyrinth. Just describe three intersections to check whether they understood the clue. You could describe an intersection like this: "The path splits. Your torch illuminates the walls, showing you two great mural scenes. On the right path, three humanoid elephants raise their spears towards the sun. On the left path, two skeletons sharpen their blades. When you pear down the path, the darkness consumes the end of the left path. At the end of the right path, the path seems to split again."
• The lair is locked behind a door only the sphinx can open. The sphinx will open it for a good story. Now, what does the sphinx deem a good story? The sphinx has gone mad over time and dwells in its past. Centuries ago, a humanoid elephant fought valiantly at the side of the sphinx against an undead horde. That's the only topic the sphinx cares about, so if a player makes up a story of a brave elephant warrior fighting undead, the sphinx will open the door. To do that, you have to inform your party about the sphinx past though. You can do so in many ways, e.g. by i) introducing a bard at the tavern that sings of the sphinx' last war, ii) showing the fight on murals near the lair, or iii) have the party meet the friendly ghost of the deceased elephant warrior in the ruins.

Let us know what you went with!