Egg
Egg is a creature subtype used for cards that depict the developing embryo stage of a bird, reptile, fish or invertebrate as an oval or round object with a chalky shell or leathery membrane. When used on creature cards they regularly turn into another creature type when a certain condition is met. When eggs are depicted on artifact cards this is not always the case.
History
The first card to use the subtype was Rukh Egg from Arabian Nights[1], followed by by Chicken Egg from Unglued. Meanwhile artifact cards like Dingus Egg (Alpha, Triassic Egg (Legends) or Chimeric Egg (Darksteel) always lacked subtypes.
When the colorshifted card Roc Egg was printed in Magic 2011, the creature type "Egg" was decommissioned and Rukh Egg was changed accordingly. As explained by Mark Gottlieb, Rukh Egg and Roc Egg were, essentially, very (very) young Birds, so they both received that creature type. Summoner's Egg was granted the creature type Egg during the Grand Creature Type Update, but it was never printed like that. With the Magic 2011 update it became a Construct.[2] The Unglued Chicken Egg was never changed, because un-types are not mentioned in the comprehensive rules.
Simarily to Roc Egg, Dragon Egg was printed as a Dragon when it first appeared in Magic 2014. The Egg subtype was reinstated when that card was reprinted in Core Set 2019, now with the subtypes Dragon Egg. Egg now can be defined more as a class than as a race.