Keen Senses. The pseudodragon has advantage on Wisdom (Perception) checks that rely on sight, hearing, or smell.

Magic Resistance. The pseudodragon has advantage on saving throws against spells and other magical effects.

Limited Telepathy. The pseudodragon can magically communicate simple ideas, emotions, and images telepathically with any creature within 100 feet of it that can understand a language.

Undead Fortitude.  If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 4) piercing damage. An the target must succeed a dc 10 constitution save. The target repeats the save at the beginning of its turn on fail. With a +2 increase. After 4 fails in a row. The target turns into a zombie. With remove curse or anyway to cure disease removes it.

Sting. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit:  8(2d4 + 4) piercing damage, and the target must succeed on a DC 11 Constitution saving throw or become poisoned for 1 minute. If the saving throw fails by 5 or more, the target falls unconscious for the same duration, or until it takes damage or another creature uses an action to shake it awake.

Swarm Mentality. Once the swarm reaches half health remove a d4 or halve the damage. An the swarm will try to stun the enemy more with sting.

Dire Disengagement. The swarm can take 2d4 damage to disengage by sacrificing its brothers.