You can always check eBay for used monster figures, especially if they're a bit older.
Any Resident Evil Hunter figure could be used for a Gregole stand in.
A Star Wars Wampa figure could be used for Ramotith
Kaiju vinyl figures could also fit the bill nicely. You'll have plenty of options ranging from Ultraman, Godzilla, Gamera, and so on.
The Neca Rhino Xenomorph figure could be a good stand in for the rhino looking zoanoid from Guyver 2: Dark Hero.
Hope these help!