Frogdog is not currently a released expansion. What you noticed on the store was most likely just the resin miniature, and not a set of rules, gear cards, settlement locations, terrain tiles, and encounters surrounding said frogdog as would be a proper expansion's loot. (ok not all of them have their own terrain and settlements, but they usually have card decks like for their hit locations and AI stuff. Still. Frogdog's just the mini right now.)

The twelve expansions that ARE released right now have been re-appearing in the shop after they get sold out with decent regularity; those ones are:

Dung Beetle Knight

Flower Knight

Green Armor Knight

Lion Knight

Lion God

Lonely Tree

Manhunter

Spidicules

Slenderman

Dragon King

Gorm

Sunstalker

And since Poots made close to 13m with his most recent kickstarter (pre-pledge-manager pledge expanding) and has been getting profits from seasonal releases ever since, I pretty much expect he'll keep putting the expansions back on the store until he dies.

Basically, you can get all the stuff now or later and it's fine. More stuff is planned (like Frogdog as an expansion) but ain't nobody got that yet. If you want to do it on the cheap, wait for what seems to be his favorite holiday - Black Friday - and loot it all then.