Item is not a defined game term, so the DM must adjudicate
In 5e, only object is someting that you could call a defined term (see this question), not even creature (see this question about what counts as a creature), and also not item.
For things that have no defined game term, we look at the dictionary defintion, here from the Cambridge dictionary for "item":
something that is part of a list or group of things
Because item is such a generic term, it is not used consistently throughout the rules but can refer to different things in different contexts (for example, the DMG has a typo that refers to the Use an Object action as "Use an Item" on page 141, there it would be equivalent with object).
Item in the context of Performance of Creation
The Performance of Creation ability of the College of Creation bard gives us some more clarity what "item" refers to in this context. It states:
create one nonmagical item of your choice in an unoccupied space within 10 feet of you. The item must appear on a surface or in a liquid that can support it. The gp value of the item can't be more than 20 times your bard level, and the item must be Medium or smaller. For examples of items you can create, see the equipment chapter of the Player's Handbook. (...)
The size of the item you can create with this feature increases by one size category when you reach 6th level (Large) and 14th level (Huge).
The additional feature Creative Crescendo at 14th level further modifies this:
At 14th level, when you use your Performance of Creation feature, you can create more than one item at once. The number of items equals your Charisma modifier (...)
You are no longer limited by gp value when creating items with Performance of Creation.
So items for the ability are things like those listed in the Equipment chapter. The equipment chapter does include predominantly objects, and also several creatures, under the Mounts and Vehicles heading on page 157. Items thus can include both objects, and at least some animals. I think this is intentional, as it would have been very easy to use the word object instead of item for the ability, if you wanted to limit it to objects. I believe the kinds of shenanigans you allude to are exactly what the authors enabled with this wording.
Some of the tables even use the word item as the column heading for the things in the table, for example the tables for adventuring gear (p 150), tools (p 154), and the ones for mounts and vehicles (p 157), which lists the animals, lending further support that these animals are "items" in the Equipment chapter. (Although the tables for Armor and Weapons use those terms as headers so I think it is more due to no overaching better term being availabel for the table; I would certainly consider armor and weapons items to be created, too).
Note that there are also services listed in the Equipment chapter (page 159). For services, you are not acquiring the creature (hirelings and such) performing the service, just work they do for you. This does not fit to the description of items in the ability, which is about physical things ("appear on a surface"). So these could not be created.
Items require a gp value, which indicates they need to be something you could purchase. The rules list no slaves and no prices for slaves, but in history during medieval times up to quite recently and even in older D&D modules, slavery was a reality, and you might have been able to buy a slave depending on price1
Balance considerations
Power-wise, creating a temporary animal is not out of bounds: there are many effects that conjure creatures, like the various spells to conjure animals, conjure woodland beings or conjure elementals. These spells tend to give you more control, as the conjured creatures are friendly to you or even controlled by you, while there is no such provision for an animal you would create with this feature. As you suggest, the animal would require some additional means by you like an Animal Handling skill check to influence its behavior.
Opposing views
In the comments to this answer two users of this site that in my opinion have a very good understanding of the rules expressed discomfort with the idea that anything should be possible to create but objects, on the grounds that creating creatures would be confusing, and or that the use of the term item in the Equipment chapter is “just a generic description of the chapter’s layout”.
While they have not put forward alternative answers to elaborate their point, I think this shows that some DMs can view this differently.
The DM decides which items can be created
Which creatures you could create as items, if any, in the end will be the decision of the DM, as the rules do not further elaborate. I would likely keep it close to the recommendation and allow the things listed in the equipment chapter, but these are given as examples, so other animals or even other creatures, including NPCs might be possible.
Tasha's Cauldron of Everything explains this nicely on page 4:
The rules of D&D cover many of the twists and turns that come up in play, but the possibilities are so vast that the rules can't cover everything. When you encounter something that the rules don't cover or if you're unsure how to interpret a rule, the DM decides how to proceed, aiming for a course that brings the most enjoyment to your whole group.
1The game provides no prices, but slaves would likely be quite expensive. Measuring Worth of a Slave states "we can assume that the unskilled wage a free employee would have received to do similar work to that of a slave is a reasonable approximation of the (...) price of a slave". Using the unskilled hireling cost per day of 2sp, over the lifespan of useful work would indicate a price of about 2,000 gp for a slave, so the bard would not be able to conjure one, until 14th level, when the gp limit is removed from the feature.
