The most USELESS monster.

I've never compared summoned creatures (from the portal) with each other. Each of them has its own functionality. And why compare them? Buy one of this type, another of that type, and a dozen of the rest. Leprechauns are simply essential for melee attacks, reducing losses by half. Satyr are often needed when you have Rangers and Mages, to give them the first turn and an attack bonus. Earth Elementals provide cover against spellcasting creatures. Valkyries, as everyone knows, are necessary to stop stompers and slow down most creatures. Air Elementals (and the Butterfly Sprite) are for quickly attacking/blocking Rangers, and sometimes they can be useful, like the Butterfly Sprite, for distracting slow infantry (Butterfly Sprites are cheaper than a Swarm of Wasps, but like a Swarm of Wasps, they can force some infantry to effectively skip a turn; more often, this infantry is turned around). Fire Elementals (Swarm of Wasps) are very often used in combination with Valkyries to break through large numbers of neutral infantry. The only criticism I can offer is about the Lights; they are removed from the game when summoned if a Swarm of Wasps is summoned. The other elementals and summoned creatures are unlikely to be interchangeable.