Sounds like despawn. Mobs that appear at night will vanish without a trace in the morning if you are 20 or so meters away.
For example - wolfs with stars will appear only at night. If you get one caged and feed it to tame you better sit right next to it stealthly for next 5 days till it tames or it'll puff into thin air. Then another 2 till it make a pup that wont count as night spawn.