I'm a little confused. You were going to pick it up on December 9th, but didn't? Then today you found a message (dated when?) that says she found it another home. Right?
I say if you missed the original pick up date, they assumed, like I would have, that they'd driven to meet you for a pick up, you never showed up, so you don't want it. I'd also be annoyed if I drove a while to meet you and you never showed up. So I wouldn't react in any way except to apologize for not letting that person know I wasn't coming to begin with.
Now, if you'd arranged a different pickup date (Jan 9th, perhaps?) and she messaged you on the 1st to say never mind, then, yeah, I'd be annoyed because if they wanted it gone sooner they should have told you so. Maybe you could have gotten it quicker or at least been given the option to say no, I can't get there that quick.