We adopted a 3 year old male retired racer (Otis) in August 2018. He had just retired from racing that June and was fresh from the track. We dealt with hookworms for several months that just cleared up in October. We also dealt with separation anxiety that included accidents in the house and destruction of furniture, but all issues have been resolved for the most part (he still has accidents occasionally when we leave the house, especially when it is dark outside). As far as his behavior, he has always been very friendly toward humans and strangers. He pretty much ignored other dogs, sniffing them briefly but otherwise not interacting. Cats mean nothing to him. Recently, however, he has begun to develop some troubling behaviors involving aggression.
A few months ago, a friend of my partner's was alone with Otis in the living room. He handed Otis a bone to chew on and then attempted to pet him on the head, and Otis lunged from a lying position and nipped him in the face. He had never done this before. He did not growl. Obviously the friend should not have attempted to get so close while Otis chewed on his bone, but the incident still shocked us and raised a bit of alarm, but we figured it was a one-off. Since then, he has shown some aggression towards other dogs on walks. When dogs and their owners are approaching us on the street, his ears and tail will stiffen and he gets eerily quiet before lunging and growling. It is clearly not playful in intention. We took him to my mother's for Thanksgiving (he had been many times before and never had any issues) but this time he lunged at a neighbor who entered the home and growled at my sister when she walked past his dog bed. He also growled at a small child who had been petting him for more than ten minutes without any issues.
At the dog park he wears a basket muzzle and this seems to help. He plays with other dogs and does not growl or show aggression. He approaches other dog owners and asks for attention and rubs. He seems in a good mood and has a good time. We assumed his behavior was improving and that socialization was helping. However, last night my partner's mother arrived from out of town to stay with us. Last year when she stayed Otis was a perfect angel and allowed her to pet him and behave normally around the house. This year, he was excited to see her when she entered the home but attempted to bite her (bit her scarf instead, thankfully) when she approached the couch on which he was sitting. I should note that he has exhibited sleep startle in the past, but each of these scenarios occurred while he was completely awake and alert.
This behavior is very troubling to us and especially to my partner, as this is his first dog. Does anyone have any experience with this bizarre new behavior pattern and have any advice? We have yet to reach out to our adoption group but are considering asking for recommendations for a trainer or behaviorist. Is there hope for this situation, or are we destined to have a dog who can't be around friends and family without a muzzle?
AFTER NOTE: He has previously been allowed on our bed and the couches, but since the aggressive behavior began we have restricted him to only his dog beds and his crate. While my partner's mother is visiting with us, he will wear his muzzle in the house. We take the muzzle off when he is confined to our bedroom to sleep at night (at the foot of the bed in his own dog bed).
