What entirely mundane creatures in D&D have fear effects? Not saying you're wrong, just can't think of any.
The aforementioned spinosaurus dinosaur?
But if you want more: Lions can frighten anyone, pterafolk (from Tomb of Annihilation) that dive on creatures and hit them with a weapon attack cause fear, the aforementioned Warlord (a legendary creature, but not a magical one), a grung variant has a poison which causes a creature to be frightened of its allies, and the liondrake (from Fizban's) similarly has a roar that frightens (and paralyzes if you fail by 5 or more). There are also some debatable edge cases, one example being yuan-ti/doppelgangers/changelings, which have a "Unsettling Visage" feature that appears to be a function of their (within-the-D&D-world) mundane anatomy as shapechangers. That is, the fear isn't magical, it's simply an expression of their fantastical anatomy. Likewise, Frightful Presence on basically any dragon of sufficient age is functionally identical to its 3.x/PF1e version, which at the time was an [Ex]traordinary ability, not [Su]pernatural nor [Sp]ell-like, so there's no reason to think it has suddenly
become supernatural in 5e.
And for those who have claimed that the Battle Master maneuvers don't apply,
Icewind Dale: Rime of the Frostmaiden features an NPC, Gunvald Halraggson, who is functionally a mid-to-high-level Fighter, with (explicitly) Second Wind, Indomitable, and the ability to make three attacks. The important bit, however, is that he has Menacing Blows, a feature which (other than being 1/round rather than X/SR) functions
identically to the Menacing Attack maneuver for the Battle Master.
So....yeah. Entirely mundane beings,
even by Earth standards, can force someone to feel fear if they fail a roll. Even some things which are not mundane by Earth standards, but which are, functionally, mundane by "we live in a fantasy world" standards, aka not using any supernatural powers just....what their biology permits, can also do it.