Are magic items for animals balanced?
The main thing to worry about is the attunement slot factor. Every creature has 3 attunement slots, meaning the Beast Master Ranger could theoretically effectively utilize 6 attunement items (See this question on whether animals can attune to magic items). The likelihood this comes up depends heavily on the number of magic items the character's will find, but to avoid all problems, you could balance the items to not require attunement.
Other than that, just make sure that you count the magic items provided for the Ranger's Companion counts against your informal count of items for each character so that the other members of the party don't feel like the rangers are getting special treatment since they have a companion.
How to generate the items
You can find guidelines for how to create homebrew magic items in the Dungeon Master's Guide (p. 284) and I strongly recommend trying to use the "Modify an Item" section if at all possible. Unfortunately, there are currently no magic items specifically for animals to base this around, but there should be no issue with modifying magic items traditionally for humanoids with some change in wording.
I'll take a stab at it
Here's how I would make these items if I decided to allow my player's to find them. (Take my ideas with a grain of salt, but they might help you understand my process):
Rune-forged Brass Knuckles
Weapon (mace), rare
This magic mace takes the form of brass knuckles which is a piece of metal shaped to fit around the knuckles with a bar on one end to strike with.
While you are wearing these knuckles, you cannot willingly open your fist. You can think the weapon's command word (this can be done even if you do not understand any language) as an action on your turn or for free if you has not interacted with an object or the environment yet on your turn (this will use your free interaction for your turn) and the knuckles will teleport from around your knuckles and into your hand (being held).
You gain a +1 bonus to attack and damage rolls made with this magic weapon. You are considered proficient with attacks made with this weapon while it is around your knuckles.
When you roll a 20 on an attack roll made with this weapon, you can have the target make a DC 15 Strength saving throw or be stunned until the end of your next turn.
(based on the Mace of Smiting item /DMG 179/; DC chosen as Dagger of Venom item /DMG 161/; rolled-20-effect based on the Stunning Strike monk feature /PHB 79/)
Orb of Bladed Teeth
Wonderous item, rare
When this small orb is placed in a willing creature's mouth, it dissolves into sharp teeth that line the inside of its mouth (covering the creature's original teeth it it has them). If the creature holds a special tea in its mouth, the teeth return to their original state and can be removed as an orb. The identify spell or a short rest spent investigating the orb or the teeth reveals the special ingredients needed to brew this tea (which cost 5 gp).
While the teeth are in a creature's mouth, it can treat them as a magic dagger. The creature is considered proficient with attacks made with this magic weapon and gains a +1 bonus to attack and damage rolls it makes with this magic weapon. If the creature already has an attack that uses its teeth, the bonus can be applied to that attack instead.
(based on the Weapon, +1 /DMG 213/; rarity increased due to its versatility and application to creatures with very powerful bite attacks)
