On rare occasions, a foal may "grow up" to be Eternal Youth. This is exactly what it sounds like. An Eternal Youth horse will retain its foal picture forever, and a special icon 
will appear on its page. Some horses, such as the Azimuth Pegasus or Clouds Alicorn, will always grow up to be Eternal Youth, and others, such as the Soft Blossoms Unicorn pictured here, will even change subtly upon gaining the status.
Since Hippocampi, Ethercorns, and Fakeicorns do not have foal art, they may never gain Eternal Youth. Additionally, some colors, mostly Silver and Gold Customs, do not have unique foal art, or in the case of some Gold customs, the creator did not pay for the possibility of Eternal Youth, so those colors do not have Eternal Youth forms either. If a color has an Eternal Youth form, there is no possible way of telling whether or not a horse will gain it until it reaches level 150.