I run custom campaigns on discord and in the latest one the adventures were deceived into working for a Baba Yaga to "recover" an items she wanted from a Bugbear chief,which they did by one of them rolling very good and managing to charm it,and even then The bugbear wanted almost everything they had in exchange for the item. Now that that they've given it ideas that the town in question is easy pickings I was working on bringing a horde down,but what monsters would be a suitable challenge for a level range of 4-7, that would fit the whole goblinoid idea? As they saw the monster sheet for the Bugbear Tyrant custom monster I was using,and claimed that they could've taken it easily,so now I want to give them more of a challenge.

Maybe some Hobgoblin Devastators?

Change some of the stats on your custom Bugbear. After all they only saw the sheet for that particular one lol

When they question you about it, tell them they only saw the stats for that Bugbear not his friends lol

There’s nothing saying that you can’t use Ogre stats and just describe them as Bugbears.

you can also use the stats for NPCs, such as a veteran or champion, then give them some bugbear traits.

You could always just give them class levels too. A group of 12 bugbears is intimidating, a group of 12 level 5 bugbear Barbarians, Druids, Rangers, and Rogues is terrifying.

The scariest villains are the ones that can do the same stuff as the PCs.