On Friday 7/27 we were adopted by Emily from the Humane Society. She's a sweet 58# 7 yo border collie mix that needed a forever home.
She's real good with the cat - mostly ignors her though Abby is still PO'd, the problem is with our little 13 yo 12# Lhasa mix, also from HS. They have had 3 set to's - 2 of them all noise but the last one (over a chewie) was getting down to business, Hannah ran screaming and limping and Emily had a small scrape and blood on her nose.
We took Hannah with us when we went to meet Emily and they were fine, but here at home it's a different environment. Hannah avoids Em and sticks close to me - always. We got Hannah when she was almost a year old.
It's pretty typical behaviour from your little dog, since she's the alpha female (so she thinks) the trick is to slowly convince her that the other dog is no threat, and to exert your dominance over her. She's had her way for 13 years; so the territorial possesive behaviour is to be expected. I would seperate the new dog , with a crate, hopefully she is crate trained, and try to introduce her into the whole house a bit slower,and when your little Lahsa acts up firmly reprimand her, but reward her for nice behaviour. A baby gate might be a useful tool as well, I just think your older dog needs time to become used to the new invader!!!! Good luck, it will get better.
We did start crating if you can call it that, using the spare room at night and if left home alone - I was feeling bad then realized air conditioned - really comfy full size bed... you get the idea. When I shower I separate them too by closing a door to front of house. (old house set up)
You know the problem is we don't know for sure just whose instigating, I think they've taken turns...
I adopted an aussie mix in May and my maltipoo is still adjusting (but it is MUCH better). What has helped a lot is that I try to spend at least 30 minutes alone with Cricket (my maltipoo) while Bella is no where close by (usually outside) so that Cricket doesn't feel like Bella is taking her place in the pack. Someone on here recommended it and it did help Cricket become a little less aggressive toward Bella. They will even sleep near each other now - not touching as Cricket still doesn't like that and Lord help us if Bella rolls over in her sleep ;) - but at first even that wasn't possible. And Bella has learned to ignore most of Crickets 'under her breath' growling.