So -- yes -- Burt Young is far more well known for his work in the wide, wide Rocky universe of films. His heartfelt turn as Rocky's best friend and brother-in-law 'Paulie' in the boxing saga is a justly celebrated work from the actor, and I'll always encourage folks to check out that Sylvester Stallone headlined work as it's arguably still one of my personal favorites. Young was around for the fights and whatnot all the way until 2006's Rocky Balboa, and he'll likely always be remembered for sticking in Rocky's corner through thick and thin.
As for genre appearances?
There are a few, and I'll always give them the attention they're owed in this space as time permits.
Some consider 1977's Twilight's Last Gleaming a Science Fiction entry. This highly-regarded Cold War thriller postulates a kinda/sorta World War III scenario, but I've always looked on it more as a political film than anything true genre specific. Granted it was set slightly in the future (shot in 1977, it takes place in 1981), so I suppose there's some small merit to its consideration, and Young enjoyed a secondary role in it ... so there, purists.
1980's Blood Beach -- a true SciFi/Horror hybrid -- came from the mind of writer/director Jeffrey Bloom; and it essentially tried to take a bit of commercial advantage to the still lingering scare of audiences to go back to the beach (thanks, Jaws, for that trend). If I remeber this one correctly, I believe Young played a police detective or something tasked with investigating some of the dark events of a subterranean terror.
In 1982, Young showed up prominently in the cast of Amityville II: The Possession, a sequel to 1979's box office success story that the true-life-story-inspired The Amityville Horror brought into being. I couldn't say anything about this one as I've never seen it. But kudos on Young for braving things that go bump in the night.
Also along the way, the actor scored visits to such entries as HBO's Tales From The Crypt serial, the 1990's incarnation of The Outer Limits, and the 2002 SciFi/Comedy The Adventures Of Pluto Nash headlined by funnyman Eddie Murphy.
Our warmest thoughts and prayers are extended to the family, friends, and fans of Burt Young.
May he forever rest in peace.