There are 2 others: Sperm Whale & Huge Giant Crab
It's easy to find the answer to this by filtering D&D Beyond's monster listing to beasts of CR 8. There are 3 results in all:
Sperm whales are common
The brief 2-line description of the sperm whale under its entry in the Icewind Dale: Rime of the Frostmaiden adventure (p. 309) doesn't mention where or how commonly it's found. However, the introduction to Appendix C: Creatures does call them "common beasts" (p. 268; emphasis mine):
Several of the creatures in this appendix are common beasts—foxes, hares, mountain goats, seals, sperm whales, and walruses—that live among other beasts from the Monster Manual, [...]
Likewise, elsewhere in the adventure, whale oil (p. 103) and whale hunters (p. 114) are mentioned as things common in the area of Icewind Dale, so it's likely reasonable to expect that characters that have lived or worked on or by the sea have seen sperm whales (or at least know of them).
That said, with their 0-ft. walking speed (and 60-ft. swim speed), a sperm whale is probably a terrible choice of creature to polymorph into unless you're fighting or traveling in the water.
Huge giant crabs... probably not as common
The Huge giant crab first appeared in the White Plume Mountain adventure from the Tales from the Yawning Portal anthology (p. 103) as a variant of the general giant crab statblock from the basic rules. Its HP is increased by an order of magnitude, its Str and Con scores are substantially higher, it gains several condition immunities (from a "rune-covered copper band"), and it has a vastly improved claw attack, increasing it from a CR of 1/8 to a CR of 8.
The same CR 8 statblock was later reprinted in Storm Lord's Wrath (the first of three digital-exclusive supplementary adventures for the D&D Essentials Kit adventure, Dragon of Icespire Peak). Here, rather than referencing the regular giant crab statblock in the text and merely listing the changes to it (as White Plume Mountain did), the huge giant crab statblock is simply printed in full in the adventure's creature list - likely because it is a digital-exclusive product, rather than an adaptation of a physical book by Wizards of the Coast. (There's no mention of the source of its condition immunities, which were previously ascribed to a "rune-covered copper band" in White Plume Mountain.)
However, in both cases, there's no real information on the commonality of huge giant crabs. White Plume Mountain merely describes a single Huge giant crab that guards a single room, and doesn't mention or imply the existence of others of its kind. Meanwhile, Storm Lord's Wrath doesn't include flavor text or lore for any of the monster statblocks in its appendix, and the body of the adventure only mentions one or two at most:
The previous guardians are not the only creatures that protect the shore in the service of Talos. A huge giant crab (see appendix A), or two crabs if there are more than four characters, including sidekicks, floats under the surface 200 feet from shore.
As only a few of these crabs are described in any source, with those in the latter adventure acting as guardians in service of the deity Talos, it seems less likely that these are common, naturally occurring creatures, and more likely that they're some sort of magically or supernaturally changed version of normal giant crabs (which are probably more common). As such, if you're looking for beasts that player characters are likely to have encountered before, the Huge giant crab is less likely to fit your expectations.
(Of course, as DM, you are empowered to determine that certain creatures are more or less common than indicated in the official books - but if you intend to run a game in the Forgotten Realms or otherwise based on official material, hopefully you find the overview above helpful.)