A DoReMi elf from Suikoden canon. They actually can fall into the realms of summons since there was a rune for them in V and individually, they are pretty pathetic monsters that try and sing people to death (and it usually does little damage). But if Hisoka gets a group of about six of them, they actually could get rather vicious with their combine attack.

Also I know we were thinking in a meme at some point to have Yuber accidentally summon a furfur which is basically a violent ball of fur that is also cute. That doesn't normally fall into his realm to summon, but about any monster in Suikoden canon could technically count as a summons with the way summons there work. I can definitely think of some more random weak joke monsters that could work if you want them. Like flying caped squirrels for instance. They actually are monsters and can be tamed by certain creatures in the canon, so that could work too.