...but not all the time? I click a spawn egg onto them, I get (for instance) one cow, and then it turns back into say a zombie or spider spawner. Sometimes. Earlier I tried a second (chicken) spawn egg after a particular one had changed back into zombie, and it permanently changed to chicken.

I will also note I've been moving these with the thing from Not Enough Wands.

This is a vanilla feature, has been pointed out before - I may change something in the future for this - but not atm.