ok thanks everyone. i know 'humping' is a display of dominence, as molli has been doing that to my puppy. (now named sammy)

but another problem. today molli bites sammy when he gets close to her will she is eating or playing. i picked molli up and she bit me!! she barks at me when i play with sammy. (although i pay equal attention to molli)
and she wont listen to me anymore. i will yell her name but all she does is try and chase the new dog. its like since i got sammy the 'devil' is coming out of molli. will she stop being so mean when she gets used to sammy??