Have you cleaned your dog’s teeth recently? Maybe you should get to that.
Alternatively, your dog may have farted. Or rolled in something bad, or eaten garbage, or been in your pond, or done any number of doglike activities. Unless you can be any more specific, best just bath your dog, clean their teeth and freshen their immediate environment generally.
I'll just wait here. Hope you've got a good lock on the bathroom door!
There. That's better, isn't it? Just smell that fresh, clean, lovely-- Ooops. Where did your nice clean dog go?
Ah. Here he is! All covered in...
Yeah. That's not going to wipe straight off, is it? And, oh dear. Is your sofa cover washable?
Still, look how happy your dog is!