List of items needeed: 1 dispencer, 1 sticky piston, redstone, spawn eggs for the mob of your choice, redstone torch and a wall.

Step one: place the dispenser a block under the wall. Put the spawn eggs in the dispenser



Step two: dig 4 blocks down in front of the dispenser and place a redstone torch at the bottom of the hole then the place the sticky piston above it.



Step 3: put a block on top of the sticky piston and on top of the dispenser to hide them.



Step 4: Go on the other side of the wall and make a redstone trail leading to the dispenser.



Step 5: Put a block at the end of the redstone trail and put a button on it.



Step 6: Press the button and YAY a mob appears!!!!