I’ve just come back to subeta for the first time in a good while but upon logging in I receive this notification:

Is there anything I can do about this?

There's sadly not much you can do, once someone has been gone for a year their account becomes fair game so to say. You could try messaging the new owner of course, explaining the situation but it's absolutely their right to keep the Pet if they want to.

when someone is gone longer than a year, their account 'clears' - meaning the user name and pet names become available. Prior to this, it used to be the entire account was deleted after a year of inactivity.

Welcome back.

You get free renames on the pets with _### rather than having to fork out 500 csc to change them.

'Evelyn.' with a dot is free - perhaps that's an idea if the name means a lot?

Evelyn was adopted in December, so that must have marked the year of your inactivity.