Rabbits can drop out of a race of course but they can't join an ongoing race. They would have to have been part of the pack.
Technically... its called "a herd of rabbits"... not a "pack". It wasn't a race really, it was an event set up to break the record. And even at that, its not an official record for various reasons. Here's a discussion. https://www.theatlantic.com/health/...b-two-hour-marathon-how-legitimate-it/599974/
Ahh now it all makes sense. But not to diminish the physical feat of running 26 miles at an average of 4:30 per mile, the course was set up in such a way for him to succeed in conditions all the other marathon runners before him never ran in: The planning that went into the event was a fantasy of perfectionism. The organizers scouted out a six-mile circuit along the Danube River that was flat, straight, and close to sea level. Parts of the road were marked with the fastest possible route, and a car guided the runners by projecting its own disco-like laser in front of them to show the correct pace. The pacesetters, a murderers’ row of Olympians and other distance stars, ran seven-at-a-time in a wind-blocking formation devised by an expert of aerodynamics. (Imagine the Mighty Ducks’ “flying V,” but reversed.) He had unfettered access to his favorite carbohydrate-rich drink, courtesy of a cyclist who rode alongside the group. And the event’s start time was scheduled within an eight-day window to ensure the best possible weather. The whole thing was as close as you can get to a mobile marathon spa treatment—if going to a spa were paired with the worst discomfort of your life. Such an extensive level of support, combined with the fact that Kipchoge wasn’t actually competing against anybody, pushed the event outside of official marathon conditions and prevented his performance from counting as a true record. I am sure many people who ran actual marathons through city streets and up and down hills will tip their hat but call bullshit he could ever do that in real race conditions.