What abilities would they have that would make them harder to defeat?
Here are few that 5e already uses:
Nimble Escape. The goblin can take the
Disengage or
Hide action as a bonus action on each of its turns.
Blurred Movement. Attack rolls against the quickling have disadvantage unless the quickling is
incapacitated or
restrained.
Evasion. If the quickling is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
Reckless. At the start of its turn, the minotaur can gain advantage on all melee weapon attack rolls it makes during that turn, but attack rolls against it have advantage until the start of its next turn.
Charge. If the minotaur moves at least 10 feet straight toward a target and then hits it with a gore attack on the same turn, the target takes an extra 9 (2d8) piercing damage. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be pushed up to 10 feet away and knocked
prone.
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.
Avoidance. If the demilich is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
Legendary Resistance (3/Day). If the demilich fails a saving throw, it can choose to succeed instead.
Rampage. When the gnoll reduces a creature to 0 hit points with a melee attack on its turn, the gnoll can take a bonus action to move up to half its speed and make a bite attack.
Longbow. Ranged Weapon Attack: +4 to hit, range 150/600 ft., one target.
Hit: 6 (1d8 + 2) piercing damage,
and the target’s speed is reduced by 10 feet until the end of its next turn.
Pack Tactics. The wolf has advantage on attack rolls against a creature if at least one of the wolf's allies is within 5 feet of the creature and the ally isn't
incapacitated.
Pounce. If the tiger moves at least 20 feet straight toward a creature and then hits it with a claw attack on the same turn, that target must succeed on a DC 13 Strength saving throw or be knocked
prone. If the target is
prone, the tiger can make one bite attack against it as a bonus action.
Magic Resistance. The slaad has advantage on saving throws against spells and other magical effects.
Stench. Any creature other than a troglodyte that starts its turn within 5 feet of the troglodyte must succeed on a DC 12 Constitution saving throw or be
poisoned until the start of the creature’s next turn. On a successful saving throw, the creature is immune to the stench of all troglodytes for 1 hour.