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.

Most monster book entries do not indicate when monsters lay eggs. If a case arises where this is relevant (such as if PCs are attempting to breed their flying mounts), the DM should select a season when that monster lays their eggs. It is recommended for game balance and simplicity that monsters who lay eggs of notable value (such as hippogriffs) do so only once a year.^Ex This selected time can also be helpful for determining if lairs (such as hippogriffs') contain eggs or fledglings.¹⁾

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 lay unfertilized eggs in the campaign setting, assume that their value has already been deducted from the animal's upkeep cost (the owner ate or sold the eggs, for example). So unfertilized eggs will effectively have no game value.