I've retrofitted the Morale mechanic into DND5 and PFRPG1 as a Will Save. I generally run HERO System, though, for both Fantasy and for Star Wars. In HERO, minions of all sorts have to make EGO[1] rolls with cumulative minuses[0] for casualties AND for any BODY damage[2] taken. Fail the EGO roll and there is often is a "collapse cascade" effect as retreating/surrendering units are considered casualties, makes the steadfast units more likely to rout as well.
Fighting hordes with one or two strong heroes, that's a good thing. You need to rout those mobs. Each additional engaged melee enemy makes it easier and easier to hit you, and before long the individually weak goblins are climbing all over you, taking off your armor (so they can shiv you to death), smear you with goblin fluids, among other indignities.
Anyone with high EGO or strong Mental Defense- which is any Big Bad Guy, unless it is a "Big Bad Also Dumb Guy" like a troll or a rancor - will be harder to turn with a Morale roll. BBADGs can also be fun since you have a chance of stealing control from the bads, with a contested Skill roll (Animal Handling, Diplomacy, Tasty Snacks, etc). I always feel bad for the cave troll in FotR, so that's fun.
This sort of thing works more or less the same in DND-based systems, just swap in Will save, and keep in mind the difference of standard deviation between 1d20 and 3d6. An impossible penalty with 3d6 is just another Tuesday with 1d20.
[0] Like most things HERO, follow the rule of doubling. -1 for every halving of the original force.
[1] i.e., Willpower
[2] BODY is what kills you, disables your bits, makes you Bleed. PAIN is equal to BLEED is equal to the hit to Morale roll. Eh, my "stacking BLEED" is a houserule so, take that with $.02, it's not stock. HERO bleed rules are for pansies.