OK here is my take on the Magnanir, just trying to reorganize the break effect. I also added a few comments/notes in red.
Magnanir
Large Beast, Unaligned
Armor Class 13 (Natural Armor)
Hit Points 45 (6d10 + 12)
Speed 60 ft.
STR | DEX | CON | INT | WIS | CHA |
18 (+4) | 12 (+1) | 14 (+2) | 2 (-3) | 11 (+1) | 8 (-1) |
Damage Resistance: cold
Damage Vulnerabilities: fire
Senses passive perception 11
Languages ——
Challenge 2 (450 XP)
Proficiency Bonus +2
Herd Tactics. The magnanir has advantage on attack rolls against a creature if at least one of the magnamir’s allies are within 5 ft of the creature and the ally isn’t incapacitated.
Charge. If the magnamir moves at least 30 feet straight toward a creature and then hits it with a horn attack on the same turn, that target takes an additional 1d8 slashing damage. If the target is a creature of medium size or smaller, it must succeed on a DC 13 Strength saving throw or be knocked prone.
Brittle Horn. The magnamir's horn can be targeted separately and has an AC 15 and 15 hit points. If the magamir's horn is reduced to 0 hit points, it can not Charge or make Horn attacks until the horn is restored to full hit points. The magamir's horn regenerates and regains all hit points after a long rest.
I moved the break effect to the traits section where I think it should be. I also simplified it and made it more impactful. If the benefit does not have a real impact there is no real reason to target the horn, it is better to just kill the monster instead. Alternately, you could use:
Brittle Horn. If the magnamir takes 10 or more damage in a single turn, its horn is shattered.
If the magamir's horn is shattered, it can not Charge or make Horn attacks until the horn is restored to full hit points. The magamir's horn regenerates and regains all hit points after a long rest.
This option removes the complexity of targeting the horn, but requires specific trigger (10 damage - on single turn).
ACTIONS
Multiattack. The magnamir makes two melee attacks.
I am not sure this is what you want. Unless it charges there is no reason to make a horn attack since the kick is so much better. I might change it to:
Multiattack. The magnamir makes two Horn attacks. It can replace one of these with a kick attack.
or
Multiattack. The magnamir makes two attacks: one with its horn and one with its kick. It can't make both attacks against the same target.
or a combo
Multiattack. The magnamir makes two Hoorn attacks. It can replace one of these attacks with a kick attack; however, it can't target the same creature with a Horn and a Kick attack.
However, perhaps with the brittle horn trait, it needs be able to kick twice if needed. If that is the case I would increase the horn or charge damage or decrease the impact of Kick.
Horn. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.
Hit. 8 (1d8 + 4) slashing damage.
Kick. Melee Weapon Attack: +6 to hit, reach 10 ft., one target.
Hit. 10 (2d6 + 3) bludgeoning damage. A creature hit by this attack must make a DC 14 Strength saving throw or be stunned until the start of its next turn.
REACTIONS
Leap. If the magnamir is attacked by a creature it can see, it leap up to 15 feet in any direction.
Perhaps this action should state that: "this movement does provoke opportunity attacks." I am also not sure if this movement means the attack misses or not. What is your intent? Is it an auto miss on the triggering attack or does it add to the AC vs that attack or something else?
Leap. If the magnamir is attacked by a creature it can see, it leap up to 15 feet in any direction without provoking opportunity attacks and the triggering attack misses the magnamir.