Big ships have me and I'm made of extra heavy steel

Zombies would probably like the look of these nuts

I set boundaries. I surround the dearly departed. I'm white on black or green. I'm sometimes seen on the sidewalk, in various colors. People use me to secure a hold on equipment.

Put a vat around it, and it'll be vacant. No church key? You'll need to be patient.

I sit between the creature below and rider above, your tush will thank me after time on a back. With a bridle and reins, you may take control, so long as you do not sit on the horn.