dog behavior with other dogs licking

Why does my dog lick and ride my older dog?

I recently rescued a stray dog. He is believed to be one and half Shih Tzu / Poodle and not repaired. Now I have 3 dogs in my house, all are fixed. 2 male shih tzu (father 6/son 2) and a female yorkie 3. The "new" dog only are trying to mount my older dog (6) and constantly licks. We've had it for 3 months and this behavior has just begun. Play with another dog (child of 2), but do not say this behavior. Also pay attention to women. The 2-year-old is the biggest, but the father is dominant in 6 of the 3. Obviously, the new dog is confused? What is happening and how I can help?

New puppy trying to be dominant. If this is really a puppy there is nothing about this sexual behavior.

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