Kecleon (5e Creature)
From D&D Wiki
small monstrosity, any alignment
Skilled Hider. Kecleon are very good hiders. Whenever the kecleon takes its hide action, it can change its colors to mimic the colors behind it, becoming nearly invisible.
Slash. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) slashing damage.
Sucker Punch. When the kecleon is targeted by a melee attack, it can take its reaction to retaliate, dealing the same amount of damage done to it but with the bludgeoning damage type. That is, if it doesn't get knocked out first.
Kecleon are the masters of disguise, able to hide in broad daylight and not be seen. They could be anywhere, all around a group of adventurers and no one would notice. They will, however, get upset if they hide too well and no one notices them. They're kind of drama queens. While it's mostly for hiding, changing their colors could be a sign that the kecleon is happy or depressed, even showing whether its healthy or not.