My 9 yr old dog started chasing feet!?

doggo ??? what is a doggo?

Sounds like this dog is of an anxious nature and that more dangerous than a dog who is outwardly agressive, as a cross terrier, it sounds more like a terrier behaviour which his 'odd behaviours' have been allowed to continue for years and you 'hope' it gets better........ bad behaviour like this doesn't stop, it gets worse, so it is not 'new behaviour' it is an extension of his normal behaviour and you need to stop it......fear nipping, feet movement motivated especially when people leave, is the sign of an stressed, anxious ( and untrained/under socialised) dog........ your dogs needs to be under your control as this is not only bad behaviour it is dangerous behaviour ...so on a lead at all times, let the lead trial so you can quickly step on the lead and have control and YOU walk the dog away from the person ( not the person walks away) .you need to first manage this behaviour and not cross your fingers an 'hope' your dog doesn't do it

YOU ASKED FOR help, yet when we give you good advice , we get, " Yes, but...."  

IT DOES NOT MATTER - that he saw the vet TWO MONTHS ago => if this behavior was not happening... or you did not bring it up to your vet -(if it were).  As Liz said (& many of us agree and practically post it WEEKLY) =>>> when a dog's "set behavior" suddenly CHANGES, you have to rule out a MEDICAL CAUSE!  If he has a medical or cognitive issue, NO AMOUNT OF TRAINING or behavior modification -will address the issue.Something neurological (like very light transient or psychomotor seizures) and/or in conjunction with worsening eye sight, may be to blame, even some types of pain could result in oddball reactions to new people.  

"A complex partial seizure will originate in the area of the brain that controls behavior and is sometimes called a psychomotor seizure. During this type of seizure, a dog’s consciousness is altered and he may exhibit bizarre behavior such as unprovoked aggression or extreme irrational fear.

Complex Partial Seizures (a.k.a. Psychomotor or Behavioral Seizures) are associated with bizarre or complex behaviors that are repeated during each seizure. People with complex partial seizures experience distortions of thought, perception or emotion (usually fear), sometimes with unusual visual, olfactory, auditory and gustatory sensations. If dogs experience the same things, it may explain the lip-smacking, chewing, fly biting, aggression, vocalization, hysterical running, cowering or hiding, in otherwise normal animals. Vomiting, diarrhea, abdominal distress, salivation, blindness, unusual thirst or appetite, and flank biting are other signs. There is an obvious lack of awareness though usually not lack of consciousness. Abnormal behaviors may last minutes or hours and can be followed by a generalized seizure."

from this link site:

http://www.canine-epilepsy-guardian-angels.com/sit...

In that the dog is a SENIOR - he may have some cognitive DECLINE that was not known or recognizable.... at the time of the vet visit. He may NOT be ABLE to "process" the visit or the new people - as he did BEFORE. You HAVE to involve your vet and possibly even get a second opinion - if you can find a vet who specializes in, or is very knowledgeable about OLDER DOG "issues".

I would see your vet. Behaviour (especially sudden) changes can mean there is something physically wrong.