In the stream with Matt Mercer, he was giving a chicken companion to another player so, I think you can give any creature as companion to a player, not sure you can customize appearance too much, but I have no idea to be honest.
You could customize all skills stat and equipment however.
You can right click any NPC or monster and tell it to "Follow the party", but that's different from a summoning spell. A summoning spell allows the player to control their summon directly, whereas this work around does not.