Researchers behind The Silph Road, a Pokémon GO subreddit, have managed to collect and analyse more than 900 Equinox event egg hatches, documenting each hatch meticulously.

Their discoveries are summarised in the following info graphic:

According to the official TSR post, these are the current Egg Pools:

  • 2km eggs: ChanseyLarvitarMareep, Oddish, Remoraid, Slowpoke, Togepi, and Tyrogue.
  • 5km eggs: Eevee, Grimer, Horsea, Houndour, Kabuto, Mankey, Omanyte, Pineco, Pinsir, Scyther, Stantler and Teddiursa.
  • 10km eggs: Aerodactyl, Dratini, Lapras, Miltank, Porygon, Snorlax and Sudowoodo.

Bold indicates a different distance group from the pre-event egg pool

Another redditor provided an explanation why other species were observed in the early hours of the event:

My theory is that double dust started just before they switched eggs. This explains all the people saying they cleared their eggs and got 10km’s with event Pokemon. If they switched it a few hours into the event, all eggs in that first few hours are after the start of the event but before the species were moved to 2km eggs.

I have yet to see any proof of species hatching from multiple egg classes and anecdotally, the dozen or so 10km eggs I have hatched have all been porygon/sudo/aerodactyl.

These hatches are relevant for Eggs picked up from 1:00 P.M. PDT on September 22 to 1:00 P.M. PDT on October 3, 2017 and it seems that not only 2KM eggs are special.