Haha this is something I actually feel very knowledgeable about
I have 'backpacking' items to strap onto my horse in the case of needed to go anywhere....I do plan on buggin in, so I dont feel the need to have everything packed to go on a long journey with my horse. I plan on mainly using my horses to carry what I wouldn't be able to for long distances...ie large amnts of water, firewood, tools, ect.
My current set up allows me to strap my BOB, hubbys' BOB, some extra camp gear, and extra water onto my 1200lb AQHA gelding....total weight for him to carry is just over 100 lbs not including water jugs, he us used to carrying crap for me, but I should practice even more with him. My big mare would be for my hubby and I to take turns riding, she weights a whopping 1600lbs and could easily carry both of us if needed. In case we need to abandon the house for a few days...we would be ready and could hide out in the woods rather easily.
...but I dont think in a large bugout situation that a horse would be the best option. Long term caring for a horse while on the move would be difficult. People are going to be desperate, a horse might look like an easy steal for either food or transportation. If someone steals your horse and all or your supplies are strapped on...youre SOL! (I do have a mini BOB for my hubby and I to carry--usually kept in the cars but we would carry in a Bug out situation, just in case
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One thing to note that a horse should only carry 20% of their body weight...safely you do not want to ruin your horses back in the middle of a trip!!! It can happen!!!! The average size horse I would guess to weigh between 1100-1300 lbs (medium sized quarter horse type) which means that they should only be carrying 220-260lbs MAX! If you are going longer distances it is not smart to push these weight limits. If I you are serious about using a horse PRACTICE is very important!! Do not expect your horse to do extraordinary things in extraordinary places. Carrying heavy people/supplies in a strange situation can be a recipe for disaster!
If you are going to be getting a horse for each person of the family-plus a horse to carry supplies...be sure to practice! I cannot stress that enough!
Training your horse is super important!
If you have a baby on the way...and are seriously considering using horses for your bug out vehicle-consider getting one that is cart/buggy trained. Having everything you would need for a baby will probably mean you have more supplies to carry along than someone who doesnt. It would also be a smoother trip (not as bumpy) for a small child. Once your little one gets older it would be easier for them to sit on a horse- rather than be carried. I guess if I were you and serious about using horses as a bug out vehicle I would start with a trustworthy cart horse....with a covered buggy (much like the Amish use...lots of storage room) but be aware you would have to stay on the rds/even terrain...carts + bumpy rds are not a pleasant experience.
Lots of other variables to consider...this is just what I have done so far