Sheer numbers could be a factor as well. SFX crowd duplication and all-CG monsters like Daleks can only do so much. A horde of barbarians, all in different clothes and wielding different weapons and moving at different speeds across uneven terrain, would be a lot harder to do on a TV budget.
Equally, an unspectacular monster could also work. H.P. Lovecraft has plenty of eldritch tentacular monstrosities, but he also has his share of things that look normal but just slightly wrong, either visually or through other senses or a "sixth sense". This is easy to describe but harder to do visually. (It's something that visual media mostly do accidentally, with the "uncanny valley" of representations that are almost human but just off enough to be unnerving.)
A quote from Douglas Adams, or as close as I can remember, refers to going from a screen *this* big with the radio to a screen -this- big with the TV, the problem being radio is such a visual medium, moreso than TV.
As has already been suggested, invisible monsters, things that just aren't *quite* right, armies of creatures the size of skyscrapers, rippling hairy tentacular beasties that would stretch the CGI-fu of the most capable team... Properly believable insectoid creatures?
The world is your oyster...
Just remember who’s standing in your way, remember every black day I ever stopped you and then—AND THEN—do the smart thing: let somebody else try first.