INSERT table row autopopulated with data from other rows?

I am in the middle of developing a simulated horse game using PHP and databases. Apparently I have just enough knowledge to get into trouble with my grand ideas! I'm looking for help/tips on my current task, creating new horse stats based on currently registered ones in a database.

I have a horse table containing data similar the following:

id  name    stat1
--  ------  -----
 1  sparky     20
 2  spunky     10

I want to create a new horse using the average of the horse 1 & 2's values contained in stat1. So horse 3 would have a stat1 value of 15 that would be automatically calculated.

Is this possible using PHP? Or can someone point me in the right direction?