Most factions can somewhat do both horde + big beasties, especially given that you can run as a Grand Alliance, but here's some standouts:
Seraphon. Skink hordes (come in a box of 24!) going through Saurus and Kroxigor as heavier infantry with big stompy dinos for your Zilla needs.
The Undead. Legions of Nagash especially, though Flesh-Eater Courts can pull it off.
Skaven. Lots of potential Horde here. Your choice for big gets... weird.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.