Plain yogurt has no added sugar and is what should be fed to a dog. (When I say "no sugar," I mean no added or processed sugar.)

The lactose sugar in yogurt isn't what's good for the dog, dogs can't even use it (and, sometimes, they're bad for the dog). In point of fact, it's the probiotics in yogurt that are good, and feeding pre-sweetened yogurts is not as good as just feeding plain yogurt.

Feeding Kefir is even better.

Jack