I have a hard time envisioning this being anything but a novelty item to impress young children at carnivals.
For years I've houseruled that you cannot get flanking unless your ally is a credible threat to the target. If a deadly rogue is fighting an ogre, and a kitty cat gets behind that ogre to give the rogue flanking, the ogre will not even bother with the cat. Instead, it will keep its attention firmly focused on the rogue. Sometimes, if the non-threatening critter can get into position without being seen, but making enough noise to be heard, then the flanked bad guy has to spend some time finiding out what is behind him, which will give flanking that first round. But then that's the end of the flanking once he finds out it's just a puny weasel.
Sure, that's a houe rule, and maybe not everyone appreciates house rules like that. But I know that if I'm walking down an alley in L.A. (yeah, I have less business being in a L.A. alley than a wood elf has being in a dank dungeon) and a mugger steps out of the shadows with a knife and demands my wallet, while a rat rustles in the garbage behind me, I really am not going to give the rat enough attention that the mugger can stab me easily.
And as for giving up my own actions in D&D, actions that could do real damage to one or more enemies, just so I can run Francis around the battlefield - not a chance. I'd just be doing less damage, being ineffective in combat, and making the fight take longer with more risk to the party since the monsters get to beat on us more.
Seems to me to make this item worthwhile, the user must be able to remain active in combat, to take his own actions so he doesn't cripple his own team by removing himself as a useful combatant.
Yeah, I know, economy of actions. I've heard the chanting, the voices raised on high praising the economy of actions. I've smelled the acrid smoke as summoning has been burned in 43 effigy.
But really, is it really all that bad? Honestly?
So your buddy finds one of these bags. For the next few fights he amuses himself summing cats and bats and badgers and scorpions. He takes his action, casts his spell, move his bat, rolls one die. Damage is not even rolled, so hit or miss, it's just one move, one die roll. That can't take more than a few seconds each round.
A half dozen combats later, when he realizes that his little furry critters aren't really accomplishing anything in combat, he stops playing with his novelty toy and economy of actions is restored.
Net cost? Maybe 10 minutes of your life while he plays with critters over the course of a half dozen combats (but probably less).
Net gain? Maybe some humor comes out of it. Maybe a troll takes a minor action to pull an Ozzy Ozborne on a summoned bat. Maybe your fighting a woolly mammoth who runs away at the first sign of a rat. Maybe your clever DM finds some humor to interject into the combat, humor because of (and probably at the expense of) these little fur balls. And maybe this magic item sees some real use, serves a purpose other than amusing toddlers at birthday parties.
Yeah, I'd pay 10 minutes of my life to make this item useful - otherwise it goes right to the Isle of Misfit Magic Items, hopefully never to be seen again.