The Ancient Cheese Block is a Pet summoning item that are dropped by Gold Mice. It spawns an Ancient Rat to follow you around. The Ancient Rat is immune to damage from enemies and despawns upon player death.
tConfig Rat and Slice of Cheese
This item and the pet it summons are both homages to removed content from the tConfig version of the Thorium Mod. In it, Rat enemies spawned in the Caverns, and dropped Slices of Cheese used to allow the Cook to move in.