Why not just have a spell that summons a random creature with at least 4|5 ATK and an ability?
Making it a permanent is really wonky for no benefit.
Having it be a permanent gives the opponent 1 turn to prepare, and / or prevent the effect (e.g. hit it with deflag)
Essentially, this would only be important if you intend it to be a nerf if you think the effect is too powerful to occur instantly.
Otherwise, just go with a spell.