some kind of Calvary mount for dwarves. I've considered horses, boars, and even bears A mount that would function well underground is preferable... I am open to homebrewed ideas!
D&D traditionally considers horses un-suitable for underground adventuring (though mules are OK according to D&D tradition, so if you want a somewhat comedic cavalry).
Riding Lizards have been a thing in D&D. Giant spider-climbing cave lizards, for instance, might be good. You could have a tactic where the dwarven cavalry charges 4-abreast down a 10x10 corridor - one on the floor, one on the cieling, one on each wall!
You could swipe Alan Dean Foster's
L'borean Riding Snake, the Cadillac of multi-rider fantasy mounts, with crossbowdwarves firing in series as the mount's sinuous movement brings them clear of their fellow riders.
Drakes - whatever exactly you picture them to be, devolved draconoids, domesticated reptiles, fantasy dinosaurs - would also be a reasonable, though less traditional, option.
Constructs could also be good, dwarves love working stone & metal, afterall, and they don't require fodder.
Also, I've considered bulettes as a possibility, would that be to OP?
Yes, but in a fun way. Maybe they could have a lesser 'tame' version, perhaps with an enlarged dorsal plate that shields the rider when tunneling?
Any underground critter could work, really, if you could picture a sufficiently brave & clever dwarf saddling it up.