This is to prevent you from inadvertently releasing a rare cubimal - there are quite a few, from not very rare to highly unlikely. You might want to hold onto them if you want to have a full collection and the trophy that goes with it, or you can sell them to another player.
One of each of the releasable series 1, 2 cubimals- this means that the achievement only requires the cubimals that can be released be released to achieve it.