iirc Rappy Egg for the longest time was something that was out of reach for most JP players as it was tied to buying physical goods (was only 10 star). It was then later added to few XH Urgents but the drop rate was horrid - and 10 star or higher eggs are untradable. When it was finally added to ARKS rappy, then it became pretty common - I was lucky to get a 14 star rappy egg drop from ARKS rappy.
NA has it on ARKS rappy right off the bat, there's many PSO2 days and they spawn for everyone even on the premium PSO2 day on the 22nd (JP didn't have this for a long time), you will eventually get it if you grind for them.
There were some tricks to farm ARKS rappy on JP, not sure if it works on NA. On one of the underground mineshaft maps, there's a flag for Rappy spawn very close to the spawn point - you just move around for like 30 seconds trying to pop it - if you can't pop it, just reload a new map and try again.
I don't remember seeing rappy egg in collection files if at all... it might have been when I took breaks from the game. But for other eggs there's a pretty much permanent 13 star egg collection file for like all pets other than rappy and the new one they just added. I wasn't playing this Easter but last year during Easter time, they were handing out 14 star and even the color variation version of pets in collection files.