Here's a small lesson time!
One uncommon pet from the same year is worth 2 common pets so I would have to add 1 common (if its a rare pet from the same year you would need to trade 2 uncommon pets)
Now about the dates
A pet from 2012 is worth 2 pets of the same rarity from 2013 (a rare cat from 2016 is worth 2 rare cats from 2017)
And the demand:
Some pets will be more likely to be accepted because they look nicer
For example a lion is worth more than a guinea pig