Hey, I feel for you, my girl is unpredicatble too..... the vast majority of the time no problem at all and then she shows me up big time without warning
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I study her body language all the time and as Dundee pointed out it really does teach you a lot. I've now sussed out what is likely to trigger her dominant/aggressive streak and sadly it is smaller dogs or timid ones
To get around this, without necessarily putting her on the lead, I spot a dog that may be her next 'victim' and call her back to my side, hold a tasty treat, click and encourage her to focus onto me....once the other dog has passed she gets another click and then the treat. This has been working really well for us and selma's more than happy to come trotting back to me rather than ploughing in.
I'll keep doing this until I feel we've made sufficient progress
I hope your behaviourist can give you some insight into why Logan dislikes certain breeds, that's peculiar given everything you've said. Good luck!!