Just need to know if mini horn is a companion/where quest is located/3 is it worth doing?

Not sure get it from follows/near the dead horse camp you get a weapon and some xp...
need banana yucca fruits

You just need about 3-5 banana yucca fruits if you do it right. Just make sure to have your follower 'stay' while you sneak, in order to get the full exp+reward. It took about 7 minutes.

hehe it took me like 4 min or so. most animals like me so didn't have to worry. kinda stunk that the mutated bears(can never remember the name) had to die. but atleast I had the gecko and dogs to help out :biggrin:

rofl i just ran all the way to the end then ran all the way back without killing any of them took me like a minute...

Is the mini horn a follower like Rex?

More like an enclave deathclaw (from fallout 3), where it follows you within a limited area\time.