No one wants to trade unicorns

Hey so I’m not that experienced in this game. I play it with my daughter. She wants the unicorn 🦄 and I’ve been trying to get one for her but no one will trade.

I was traded two alicorns and some other random pet for a fly and a ride potion but no one wants to trade for the unicorn even though I see players with them pretty frequently.

What does it take? And why is it so hard to trade for these (or have I just been unlucky)?