Brutal Claws.  When entering combat, the front two legs of a Zergrathu (which mimic humanoid hands) split and tear, revealing shards of bone that they use to attack in melee.

Adaptive Combat.  At the end of its turn, a Zergrathu rolls a d4, which determines an effect.

(3-4).  The Zergrathu can choose to either regen 2 HP instantly, or deal 2 extra poison damage on their next hit.

Get Over Here.  A Zegrathu can use its attack to launch a spine at a target, dealing 2d4 piercing + 1d4 poison damage on a successful hit.  The target must make a DC 12 Dex saving throw if hit.  On a fail the spine gets stuck (the target can roll a DC 12 Strength saving throw to pull themselves off the spine at the end of their turn).  The Zegrathu can use its bonus action to attempt to pull the impaled creature 10ft towards it and if this causes the creature to meet the Zegrathu, may immediately attack that creature.  To resist, the target must make a strength saving throw against the Zegrathu's Strength check.  However once a creature is impaled the Zegrathu's speed becomes 0.

Get Over There.  If there are multiple Zergrathi, the one that has impaled a creature may instead throw the creature 10ft towards an ally.  However, this results in a -2 to its Strength check.  The ally does also get to immediately attack the creature if the creature meets it.

Multiattack.  The Zergrathu makes two attacks with the front bone claws, or use it's spine-shot abilities.

Bone Claw.  Melee weapon attack: +4 to hit, reach 5 ft., max two creatures.  Hit: 4 (1d6+2) piercing damage.

Quick Thinking (1/round).  A Zegrathu can use its bonus action to roll an Intelligence saving throw against one attack from an attacker it can see (so long as the attack must be rolled to hit).  If it beats the attack roll, the damage from the attack is halved.