The dog's name is Pero, the twins' mascot; but I'm not sure what its tag should be called.
Creating a mascot tag usually isn't a problem, since the tag format is "Mascot_Name_(Vtuber_Name)", but Pero is two vtubers' shared mascot, instead of one vtuber's mascot. I think a [strike]good[/strike] serviceable tag would be "Pero_(Fuwamoco)" or perhaps "Pero_(Fuwamoco_Abyssgard)", but in any case I'm not 100% sure what its tag should be, so I'm not creating a tag for it.
The creature on Pero's head looks to be Bijou's as-yet-unnamed mascot. Both mascots need character tags.
The two are consistently referred to as just Fuwamoco without the surname so I think that would work. However, the Debut PV calls the creature Perroccino. Pero appears to be a nickname the fans have given it.
Hm, so whichever tag is used will have "fuwamoco" in the brackets. Seems reasonable.
But we have to pick one. This one is tagged as Pero, and the child post is tagged Perroccino. I just rewatched the Advent announcement trailer and the 30s Fuwamoco video, and neither showed their mascot's name that I could see.
As for the critter on his head, that's not Bijou's mascot. Alas, it's Yorick, Shiorin's mascot. Tagging (but leaving the char request tag until Pero's tag is resolved)