A value of -1 means they owned the Pet but Released it and have not yet rehatched it.
A value of "null" means they owned the Mount but Released it and have not yet retamed it.
When there are 0 of these items, we can't tell if they had been owned and were all used, or have never been owned.
A value of true means they own the item now and can wear it. A value of false means they used to own it but lost it from Death (or an old Rebirth).