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A special episode of the podcast 'Monster Man' was centered on monsters that are associated with a season - winter, specifically. And I thought the idea of having seasonal monsters would be genuinely interesting. We associate plenty of flora, fauna, and nebulous concepts with particular seasons, so condensing these to traits we can apply to creatures in a TTRPG world seems to me like a way to support these assumptions and your world's inherent folklore. So here are my ideas for spring. Entries marked with * are traits or actions that already exist in 5e, and which I thought were thematically appropriate.
Below the cut is a list of things I associate with spring, and thus what I've based the traits on this list on. I should also note that this is meant for D&D 5e, but please adapt and change anything you like for your game.
Spring Monster Traits
Breezy. The creature can cast the Blur spell without having to maintain concentration on it. Definitely should be a (1/day) or otherwise restricted.
Chameleon Skin. The creature can change the color of its skin at will, giving it advantage on Stealth checks.
Early Frost. The creature is vulnerable to cold damage.
Dancer. When engaged in melee combat with only one other creature (a dance partner, if you will), this creature can use its reaction to attempt to "dance" out of the way of an incoming attack by rolling 1d6 and adding its to its AC for that turn only. The creature then moves 5 ft in any direction, provoking attacks of opportunity as normal.
Fickle. The creature can cast Mirror Image as a reaction. Use with caution - you may want to make this a rechargeable ability, similar to a dragon's breath weapon.
Intoxicating Touch. When your bare skin makes contact with this creature, you must roll a Constitution saving throw or be poisoned (or charmed, DM's discretion!). You can repeat the saving throw at the end of each of your turns, ending the effect on a successful save.
Jokester. The creature can cast Tasha's Hideous Laughter as a bonus action. Perhaps make this a (3/day) thing, or otherwise restrict your use of it.
*Magic Resistance. The creature has advantage on saving throws against spells and other magical effects.
Naïve. This creature is not yet wise in the ways of the world. Unless you are openly hostile to it, it has disadvantage on Insight checks against and Wisdom saving throws it is made to do by you or your companions.
Nesting. The creature has young nearby. It will try to lure attackers away from them, and fight to the death if necessary.
Powdery. When this creature is hit with an attack that deals bludgeoning damage, other creatures within 10 ft. must succeed on a Constitution saving throw or spend their next turn violently sneezing (= unable to take actions or speak/cast spells with verbal components). Any creature that also has the Powdery trait, or doesn't need to breathe, is immune to this effect.
*Regeneration. The creature regains 10 hit points at the start of its turn. If the creature takes acid or fire damage, this trait doesn't function until the end of its next turn. The creature dies only if its starts its turn with 0 hit points and doesn't regenerate.
Secretive. The area around the creature's lair or nest is changed to suit its whims, as per the Hallucinatory Terrain spell. This effect cannot be dispelled. It ends a week after the creature has moved away from the area, or is slain.
*Split. When a creature that is medium or larger is subjected to lightning or slashing damage, it splits into two new creatures if it has at least 10 hit points. Each new creature has hit points equal to half the original creature's. Each of them are a size smaller than the original creature.