Microsoft Windows [Version 10.0.15063]
Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz, 3.40 GHz
64-bit operating system, x64-based processor; 8.00 GB RAM
NVIDIA GeForce GT 630; GeForce Game Ready Driver Version 385.28
Java 1.8.0_144-b01 (64-bit)
If a player right-clicks an animal with its spawn egg while it's playing its death animation, the game will spawn a baby animal.
MC-1246, a mob's hitbox doesn't instantly disappear upon death. While a mob is playing its death animation, its hitbox is still there. The game probably doesn't check if the right-clicked animal is still alive or not and therefore spawns a baby animal.
- Place a repeating and always active command block with the command /kill @e[type=pig] somewhere.
- Spawn a pig via a spawn egg and keep the right mouse button pressed.
- You will see a lot of baby pigs appear and instantly die until the death animation of the initially spawned adult pig ends.