So my friends and I are trying to fight the Rhino bosses but its not working. We tried this once earlier in the world and they spawned in, however we got beaten pretty bad and retreated. We want to reattempt them now but for some reason, they are not spawning in. We place a Peach at the altar in spring but nothing happens. We're not sure what is going on. Please Help.

UPDATE: I realized that I think it may be the altar in general that won't work. Other Modded bosses will spawn such as the bear and the metal bird. However after testing this out on a separate world, Both Altar Bosses will not work. I translated the instructions and followed accordingly only for nothing to happen. I do not know what happened and is there any way to fix this issue.