OH dear me, we've got a new problem emerging with murphy 19 month old rescued springer spaniel. (rescued at 10 months.) His behaviour with other dogs is unpredictable as it is, sometimes he is good other times not, but we have found that although he tends to lunge aggressively as soon as we get level with another dog if the other dog doesnt react he is fine. (Only select dogs i know so far. ) However i am finding that he seems to react differntly to very little dogs that move fast. He has a very high prey drive with little things like birds, pheasants and cats, but is fine with big things that move like bikes, joggers and horses. (Hes a strange lad!) Anyway i have found he reacts to little dogs like he does a cat. Hes always on lead around them and is absolutely fine with them if they are calm (and not male does he dislikes anyway. ) He knows my next door neighbours young jack russell very well, he has known her since she was a tiny puppy, but just recently she is letting lucy off lead in the field where we walk. When we meet ellie (my retreiver) and lucy often play chase or lucy will just run round in circles. WHen she goes past murphy at high speed he tends to lunge out and mouth her and i'm not entirely convinced its play. After a while he gets himself whipped up into a frenzy and starts screaming in frustration. I'd love to be able to let him off to have a play as they are quite close in age, but i just can't risk it. I have bought him a muzzle, but even then if he goes into prey drive he could still do some damage. 
I am trying to transfer his chasing onto a ball, and he is started to get a bit ball obsessed. We started flyball training this week and he was a star. He was fine with all the big dogs but when a really good little dog ran he seemed far too focused on the little dog for my liking. (I'm trying to distract him when he runs.) Is there anything i can do to cure him of this problem or is it always going to be there? I know greys and lurchers have this problem but never heard of a springer doing it! I might also add that he does live with 3 cats, hes kept seperated, although they have met on occaisions and no injuries occured.
Forgot to add, hes met quite a few little dogs before, but they don't tend to dash around that much, although when we first got him he did go walking (onlead) with a puppy jack russell that was quite speedy offlead, so this behaviour is quite new.) He was fine woth other dogs up until he was castrated at 15 months.


    

It seems to me he is wanting to play fight with the very excitable little dog, sometimes it can look like they are wanting to attack when they are not. What is his body language like when he lunges and mouths?

From what you have said it seems the little one gets over excited and your dog then also gets overexcited, you can try to calm them a little when they are both playing by getting them both to come to you and hand them treats if they calm. A long tug rope can also be useful as they can then play tug with each other.

Spitzed x

If your dog is getting ball obsessed this will help enormously to redirect his prey drive and the dog should ignore other dogs if he is TRULY ball mad.  By this I mean, the truly ball obsessed dog will follow a robot as long as it has a ball in its hand and will never give up searching for a ball and will ignore any other distraction.