Sounds like a Mimic to me.
OOOOooohhh, i totaly forgot about it.
But with this idea you could make a TRAP. Like you summon a Mimic, it plays dead )) , then some bandits spot it and think: TREASURE!!
They come closer, and when they open it, BAM, it attacks them and then you join the fray ))

It would work as some kind of summon,then an illusion might (or not(depends on skill level and some %)) affect nearby humanoid to a sertain level to come closer and open the chest.
also, i don't know if it is possible in vanilla, but could you reanimate dragons? Or maybe some quest (for necromancy purposes) to MAKE a zombie dragon companion creature? Like you kill a few dragons, collect all parts (a leg from one, head from another,etc) and then in an underground laboratory you sew it together, and then after a rainy lightning storm you shout "IT'S ALIVE".
Then you could kill more powerful dragons for better parts and different breaths (if you kill fire and earth dragons, collect their glands(?) and then your pet dragon can do a magma breath)