Yellow pus coming out of a dog's mouth is usually a sign of a respiratory or digestive issue or irritation. Often, pus is expelled from the respiratory system through the nose mouth, and even eyes. Fluid from the digestive system may also appear yellow.
Possible causes include:
- Kennel Cough
- Allergies
- Ulcers
- Infection
- Silvia disorder
- Tumours
- Poison
As the pus is yellow, I would suspect that it is more likely to be the result of an infection. If that is the case it is important that you get your dog treated right away to prevent it from worsening or spreading.
Usually symptoms present themselves in pairs, so have a think about your dog's behaviour and decide if there are any other symptoms which are present. For example, is you dog drink and eating properly? If not, this is almost a sure sign that something is wrong.
Here is a helpful video which suggests some of the ways that you can tell if your dog is poorly:
Although the issue may pass, or maybe the result of something relatively harmless, you should definitely see your vet sooner rather than later, even if just for peace of mind.