Like most Magic: The Gathering cards, the value of Relentless Rats depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Secret Lair Drop)
In an Amber-focused deck where overwhelming the enemy with cards is a priority, this one is annoying to deal with in a way that other one-drop cards just can't claim
There's something to be said about never having to ink a card from your hand again, which is why the Legendary variant we mentioned earlier is also spiking at decent value since launch (sitting at around $26), but the Iconic rare is just on another planet
Summoning sickness covers all permanents and creatures are the ones who suffer the consequences of it while other permanent types dont have to worry
They're reskinned versions, with the same rules text, but in-universe MTG art and names