Bob was adopted at 7 weeks (I know a little young, but that was when rescuers put him up). He's 9-1/2 weeks old now. He' s really a sweetie, but he can sometimes react aggressively to touch or authority.
Part of the problem I know stems from his rough-house play with Ginger (our 6 year old Beagle). They just play and wrestle for hours and would do it all day if we let them. Of course, in their play fighting, there is a lot of growling, mouthing and pinning - if you will, but they always seem to know their boundaries and what is "too rough". When Bob crosses the line, Ginger lets him know real quick.
Lately, Bob has taken to growling when he is picked up. Especilly by Lizzie, our youngest girl (11 Years old). It is obvious that Bob sees her as another pup and wants to assert authority over her and has even nipped her. Bob is fine when he is separated from Ginger all day (as is sometimes the case - I take either Bob or Ginger to work with me often - but never both). These days he is perfectly sweet and will accept any and all attention with no effort to wrestle, bite or growl. When he is the "home" dog, Lizzie comes home from school, opens Bob's training crate, picks him up, plays with him and takes him out to do his business. Bob is "so happy" to see her and is not even the least bit aggressive.
Recently Bob has even been aggressive when being touched (either to pat or pickup) when he is napping on rug. A hand on his back and he will awaken abruptly and do his best mean growl and sometimes snap. Then he calms down again usually.
What's up? I usually only ever had a lone or singletary pup and am not familar with the socilaization and training of a pup introduced into a house with an already mature, but playful dog.
P.S. Ginger never, ever nipped or bit and still doesn't/won't. She mouthed a lot and chewed on your fingers and sometimes mistakenly bit too hard but was trained out of it easily as she matured. To this day she is gentle and actually takes a lot of well intended over-loving ("Smothering" Hugs, Lying down with and overly close snuggling) from Liz. She does sometimes low growl to express her annoyance, but almost rolls her eyes as it is going on) and simply lays there.