There's a couple ways you can do this. The first I've done, the second is in theory since I haven't tried it yet:
1. Use the testingcheatsenabled true cheat and then click on the horse you want to add to your household (use your camera to scan over to the appropriate ranch so you see it). Shift+click on the horse and you should see the option to Add to Active Household. Presto, the horse is transferred. This is what I did with Orion.
2. Use Edit Town. Click on the household with the horse. Split household and evict the horse and then merge it with the new household. I haven't tried this method yet so if someone does, please let me know how it goes.
I'm also trying to figure out how to save a household with a horse I'm 'selling' so that I can upload it to the Swap Shop and they retain their skills. I'll have to save them as a household but you can't have a horse alone in a household. So I might make a very small lot with a 'groom' on it. Move the horse to that household, then save the household for sharing. It would mean if someone wants that horse, they would have to insert the household with the groom but then move the horse to the intended home and then ditch the horse groom. That's the only way I see that working.
Thanks for the compliment on the house. I've always had a passion for houseplans. Long before Sims, I'd love to buy houseplan magazines and just look through them and seeing which ones I liked and how they'd best work. Now I do the same thing online but with Sims, I can build them! Almost every house I've built has been inspired by a design I've found online. Often I've had to alter some things to better fit the Sims with regard to building restrictions and sim pathing. But the inspiration is there. Here's the log cabin I used as inspiration for Thomas's house:
http://www.greatland-loghomes.com/plans/plans1.php?plan=KingsvilleI also use tons of custom content which is why I don't do much in the Shop and Swap. I find the game content too limiting with doors and windows, in particular. It drives me nuts to not have matching windows and doors. So I often get custom content where the designer has 'completed' the EA content with matching doors and windows to ones EA started. Like the matching double door for a single door in EA content. Or all the other windows you'd want to match one in EA content. I like things to match.
And stairs. Bless the stars but why can't we have the modular stair foundation piece back from Sims 2, OR use the spiral stairs idea and make half landing stairs (which someone has recently done with custom content, praise be!). It's annoying to have to use building cheats to try to make half-landing stairs because then you're limited on wall coverings or they're warped. A half-wall height interior foundation piece would make me dance in joy for days. <laughs>
I'm thinking of setting myself a challenge of trying to build the following house based solely on pictures with no floorplan.
http://www.seddoncc.com/portfolio/english_manor.html But it would be lovely!