Will pay for this....

[!Alive]

Could you explain in a more clear fashion exactly what you are trying to accomplish?
If I understand in the slightest you're wanting that when a user clicks on an adoptable they have a chance of finding/adopting an adoptable for their own. And that some of the prize adoptables are more rare to find. Right.
If this is kind of what you are wanting I may be able to help. I have a similar sort of random prize/adoptable getting system that works for me. There is just a random chance of stumbling across an adoptable just by browsing. It shouldn't be too hard to make so certain adoptables are more rare than others.