In the final rules it's pretty clear: a monster can make multiple attacks only if it has the "Multiattack" action, which always specifies which attacks can be made.
In the case of the Owlbear, it has a Beak and a Claws attack (Claws counts as only one attack). It also has a Multiattack action saying "The owlbear makes two attacks: one with its beak and one with its claws."
If a creature has different attacks but doesn't have Multiattack it has to choose which one to use at each turn.
In 5E, the design standard seems be that almost all attacks - especially weapon attacks - feature a separate roll for each target. An “attack roll” in 5E is by convention a single attack vs a single target. Multiple target attacks generally have separate attack rolls made by the attacker (as in the case of multiattack actions), but if not, there's usually no attack roll and instead each target makes a saving throw to avoid some or all of the effect of the attack.
For melee attacks, there are two exceptions: the variant "Cleaving through Creatures" rules in the DMG and the Battle Master's Sweeping Attack manoeuvre, both of which allow an attacker to target a second creature as if making a second attack, but use the result of the initial attack roll. Both have limitations on when they can be used, though, and aside from reusing the initial roll, both are a second, separate attack. (Thanks to KorvinStarmast and Slagmoth for pointing these out.)
There are more examples of single attack vs multiple targets when it comes to spells: auto-hit spells like Magic Missile, which forgo a roll altogether (and as a result cannot critically hit), and unusual spells like Sleep or Color Spray which have the caster roll for a number of hit points of creatures who will be affected (but these spells don’t inflict damage).
An attack that hits more than one target with a single roll has several strengths and drawbacks that are not already present in the system:
- a single roll will be compared against multiple, potentially different armour classes - not gamebreaking, but it may remove some tactical options;
- it allows a creature to potentially score multiple critical hits with a single die roll - something not intended by the system;
- most importantly, there may be circumstances in which advantage or disadvantage applies against one or more targets, but not all - in which case, do you apply it to the roll or not? Rules for advantage and disadvantage may not allow for this case, and may become more or less powerful than intended depending on how you rule this.
I think these are all reasons why this was not a feature included in the game design, and so I’d be cautious about adding in your homebrew creations.
Best Answer
You're right that Miss outcomes are uncommon
I've found a few more using the search function on DnD Beyond, but there aren't many.
In addition to the Piercer, there's also:
A Kobold Inventor which has a Miss effect when it throws a Green Slime Pot as part of its Weapon Invention action. Published in Volo's Guide to Monsters.
Zorak Lightdrinker who has a Hit or Miss effect after making a weapon attack. Published in Waterdeep: Dungeon of the Mad Mage.
A Fluxcharger which has a Hit or Miss effect when it uses its Arc Lightning action. Published in Guildmasters' Guide to Ravnica.