Where'd Chen get a spear from?

Who knows but it's part of the Hakumen no Mono reference from the previous page.

Isn't that spear from Ushio to Tora?

Yes, it is.

Pixiv Description:

There were two hot pot sets, so we never thought both would run out of gas. They didn't bring some of the drinks we ordered, either, so I figured there was enough to forget.

Also, one user-added tag for the Spear of Beasts is "Spear of Cheeeeeeen".

ANGRY MOB IS ANGRY AND WANTS TO EAT

with 7 people hungry, without vetables are really zombies vegetable thirsty

At this point half the beast spear tags on danbooru have it in the hands of Touhous.