You appear to have found a hacked spawn egg that a server operator dropped, or one randomly spawned by a server plugin. The problem I perceive with the other aswers is that they suggest that this egg may spawn a Skeleton or Ghast. It will not spawn any normal mobs because you can see in the screenshot that the name is "Spawn". A mob egg would have a different texture and a different name, like "Spawn Skeleton". Also, the texture is unusually white - slightly different than one of a Skeleton or Ghast. In conclusion, this is an unofficial spawn egg produced by and only usable with a specific server plugin, of which there are many. Right-clicking the egg onto the ground should produce the desired effect, although I am unsure what that effect will be. Try asking other players or the server operators before you try it. Source: http://www.minecraftwiki.net/wiki/Spawn_Egg

You appear to have found a hacked spawn egg that a server operator dropped, or one randomly spawned by a server plugin. The problem I perceive with the other aswers is that they suggest that this egg may spawn a Skeleton or Ghast. It will not spawn any normal mobs because you can see in the screenshot that the name is "Spawn". A mob egg would have a different texture and a different name, like "Spawn Skeleton". Also, the texture is unusually white - slightly different than one of a Skeleton or Ghast. In conclusion, this is an unofficial spawn egg produced by and only usable with a specific server plugin, of which there are many. Right-clicking the egg onto the ground should produce the desired effect, although I am unsure what that effect will be. Try asking other players or the server operators before you try it. Source: https://minecraft.wiki/w/Spawn_Egg