Hi Rainbowandyroo.
Timothy sounds like a proper little character - just right for a cat of his stature.
Unfortunately, the incident with the dog fight has clearly unsettled him, and what he's now doing is redirecting the fear and aggression he experienced from the incident at your own dogs, his companions.
Does the aggression only take place when he is out on your walks with the dogs, or does it happen in the house as well?
This kind of thing can often happen between cats too, who have otherwise lived peaceably and happily side by side until something happens which shakes their confidence, then one cat can get bullied something rotten by it's housemate.
Usually the way to address it is to separate the two animals concerned, and keep them isolated for a while, gradually reintroducing with techniques such as site swapping (ie rubbing each pet with a cloth or glove or sock, and then swapping the scents over, letting each cat have the other cat's cloth to smell.) and sight blocking (making sure the animals can't see each other.
If this behaviour only happens when you're out with the dogs, then you may have to consider curtailing Timothy's allotment excursions for the foreseeable future.