First April Fool's Event was taking place in 2011. Users were pranked by Magistream with the most amazing egg that has ever existed and ever will exist. Creatures released during this event are usually a joke, like non hatching egg or a piece of rock. During April Fool's 2012 there was a big snowball fight between users.
The most amazing egg appeared in a Stream
One new creature and a snowball fight
Each user received 10 snowballs. They were also available for 100 at Remy's Inn. When a user threw a snowball at another user, the other user could keep it. The maximum amount of snowballs one user could gather was 30. On April 2, all snowballs in circulation were changed to snow dragon eggs and unfrozen. Remy's Inn also stopped selling snowballs.
Tristan announced that Magistream will change into a trading card game - MagiStream The Gathering and all of the eggs will be sold as trading cards only. What's more, the cards will be obtainable with crystal shards, not "annoying, hard-to-get gold". The original announcement can be seen here.
During the 2014 April Fool's event there were no new creatures released nor any re-release of Oblivion Pillagebeasts.
Normally unsuccessful breeding resulted in obtaining a Bread Creature.
Even if those creatures seem uninterested in each other, breading them was successful! Whoopie doo! Eagerly you dash to see the results.
Easter eggs (Painted and Flecked Sceaps), that were still available in the Stream, still looked the same, but on April 1st they turned into a new creature. The creature counter didn't went up at first, to confuse Magistream users. Later it went up and the creature's name was changed (to Lunaries).