The NCDL (or whatever they're called now)....Dogs Trust I think...are brilliant for sponsering dogs. It's only about Â£5 a month and you get all sorts of lovely things from your sponser dog. Also, you can go and visit them and take them for a walk if you pre book it.
When i was about 12 my mum sponsored a cat for me from he cats protection league as a gift and that was lovely you got a photo, stickers a gift. Plus a magazine that was really good every couple of months.