Dog aggression - 5 months old

My friend has recently given a home to a 5 month old pup, and he brought her round to see me. He wasn't sure how she would react to my dogs so he kept her outside, she was lovely, very friendly and waggy tailed. He said that when she sees another dog, she rears up and lunges towards towards it and isnt sure whether shes been friendly or not and wanted my opinion, so I got my Ruby out (we used her as a guinea pig bless her!) I took her outside and we let them approach each other, Ruby was her usual friendly self and my friends dog just changed immediately.

Her whole body language changed, she went stiff as a board, her tail was right up, her ears pricked forward and she bared bared her teeth and flew at Ruby, she never got near her as I could tell immediately from her body language that she wasn't going to be friendly.

Someone has told my friend that his dogs mother was either a pitbull or part pitbull and was recently PTS for killing another dog. How much of this is true, I dont know - you know how some people like to exaggerate I dont know much about these "types" you know what I mean...

How normal is it for a 5 month old puppy to be aggressive? I was really shocked by it I expected her to be all bouncy and wanting to play etc. Any advice/opinions?

Thanks x

hi

difficult as wasnt there to see the body language,but sounds like it may of been fear...has she been socialised at all?

Hi, thanks for the reply

I think she has been socialised, I have seen her out and about with her previous owner and she did live with another dog before.

It may have been fear but she was very "forward" if thats the right way to describe it. When I took Ruby outside, I stood still with her and let the other dog do the approaching if you will. If that helps?