A friend of mine discovered this recently when we were playing Heartgold and Soulsilver. En route to the Safari Zone (Route 47), you'll find some trainers, a Young Couple called Eda & Duff, who have a Cloyster and an Onix, respectively.
For point of reference, here's an Onix:
... and here's a Cloyster:
The two Pokemon have nothing in common, so why would a lovey-dovey Young Couple have this specific pair? I'll leave it to you to discuss.
For point of reference, here's an Onix:

... and here's a Cloyster:

The two Pokemon have nothing in common, so why would a lovey-dovey Young Couple have this specific pair? I'll leave it to you to discuss.