[Uatu]

One other thing we could do, now that you mention the hyaenodon's different lineage, is to use specific prehistoric animals for "giant" versions of animals in the Monster Manuals, i.e.:

Ant, Giant
Badger, Giant
Crayfish, Giant
Crab, Giant
Crocodile, Giant
Eel, Giant
Gar, Giant
Octopus, Giant
Owl, Giant
Pike, Giant
Ram, Giant
Rat, Giant (Sumatran)
Scorpion, Giant
Sea Horse, Giant
Skunk, Giant
Slug, Giant
Stag, Giant
Toad, Giant
Turtle, Sea, Giant
Turtle, Snapping, Giant
Weasel, Giant
Crane, Giant
Dragonfly, Giant
Mantis, Giant
Raven, Giant
Solifugid, Giant
Vulture, Giant

...of course, not all of these are suitable for conversion (some have still-extant versions that would be fine while others are just too much of a stretch)...

Also, Megalodon - ha ha, actually I will have to go and fix that one of these days... Wish we knew what color those things were (as of now it is just the same colors as its closest living relative, the Great White Shark).