Some monsters such as the hippogriff lay eggs. The eggs listed in the MM as having value are assumed to be fertilized, since it is assumed that's where their value lies. Therefore, eggs found in lairs as part of the numbers listed in the MM will be fertilized.

For simplicity's sake, it's recommended to ignore biological questions of whether monsters lay unfertilized eggs, and how much such eggs are worth. If a monster in captivity, such as a flying mount, does biologically give birth to unfertilized eggs in the campaign setting, assume that their value has already been deducted from the animal's upkeep cost. So unfertilized eggs will effectively have no game value.