Legendary Week was a unique event in Pokémon GO that took place in the latter half of February, 2018, and resulted with another mini-event.
During the event trainers had opportunity to catch all three legendary Pokémon of Weather Trio, as for the time of the event Kyogre and Groudon had returned to the Raid Battles to be battled along with Rayquaza that was available at that time.
Moreover, if Rayquaza was defeated more than Kyogre and Groudon combined, Pokémon that typically prefer windy weather (for example, Bagon) would be more likely to hatch from Eggs from March 5th to March 16th. If not, Pokémon that prefer sunny or rainy weather (like Trapinch or Lotad) would be more likely to hatch. 
Pokémon Eggs hatchlings
The results given on March 5th, 2018, stated that Rayquaza was defeated more times than Kyogre and Groudon combined, which resulted with Pokémon preferring windy weather hatched from Eggs. 
Following Pokémon were more probable to hatch from Eggs obtained between March 5th to March 16th, 2018: