Hey! For the build item, they are different depending of which upgrade you have bought : one is a tower that attract human, one an archer tower, one explosives and one is a bumper which push humans in a direction (probably the one you encountered!)
For the big undead monster, we kinda already have one !! xD If you take the right upgrade in the book you can unlock a "giant" skeleton which is pretty big and powerfull!
hope it helps !