Boxers usually love to run, and since they're
pretty fast and agile they often will volunteer
to be the "prey." Once they have a running buddy
of the same speed and skill level, it's a treat to
watch them hand off the "it" position and switch
places. Unless the dog is running home or to
some other safe spot, I wouldn't be concerned
(Presumably this is happening inside a fenced area
or very isolated from roads)
Wait until the first heavy snowfall and take your
dog out to run in it -- you will see him/her run
with incredible joyful abandon, even alone.
At 6-8 months, puppies are entering a transition
stage where older dogs will tolerate less and less
disrespect from them, and often appear to "crack down"
on them for no reason at all. As long as the pup isn't
being harmed, allow the older dog to put him/her in
place. Chances are the pup will bounce back up to
his feet and offer to play again after being chastised,
but some serious "talking to" is to be expected in the
near future.
Adult dogs are very tolerant of exhuberant young
puppies, but when they hit 6-8 months it's not so
amusing to them any more and they get cranky. The
party's over -- time to act like a grownup dog!