Switch to Fone Bone and play that strength test game. the viking hemet will get knocked off and the possums wil play with it and run off. now switch to Smily Bone and put the red dye in the pot of soup. now do you see a rag hanign near th' door? grab it an' dip it in the pot. it will turn red. go outside and hang the rag on the tree near the helmet and get the possums to play with it. when there done they will let you have the helmet. finally, last but not least take the helmet and use it on your cow suit. tada! You have your horns! Good luck!
P.S. Oh but first you gotta use phoney too. talk to Jon and concince him to bet on the race. now switch to Fone Bone and wait until Jon talks to the strong guy(who, if you ask me, sounds an awful lot like StrongBad) Now you can ajust the strength and do the above