I own horses, and my husband worked at a fodder store for years so has lots of experience from the other side of the fence.
It depends on the horse and the feed, along with how much work the horse is in.
Often people try to blame the feed when it is actually their riding and training causing issues. But definitely you don’t need to feed a high energy feed to a horse that isn’t in work. Really high energy feeds aren’t useful for any horse unless it is competing and training heavily. Those feeds if fed correctly need to be fed in very high amounts to meet the nutritional needs of the horse. None of this “1 scoop” nonsense – feed needs to be weighed and fed according to the packet.
Then it can depend how the horse reacts to feeds. I have one who will go absolutely off his rocker (think child with ADHD on a sugar high) if fed anything soy based – even a handful of pellets. But I can load him up on high starch grains like corn and oats, and he is absolutely fine. Wheat has that effect on a lot of horses, which many owners don’t realise.
I have a mare who can be pretty nutty at times, but a lot of that comes down to hormones. Thankfully with her raspberry leaves and chaste tree berries (powdered for both) work really well. I don’t like using chamomile which is a great calming herb because it is a banned substance at shows and they do test for it.
I do think the majority of horses are fed a higher energy feed than needed. I go for as much hay as they need (I have never had to limit for an overweight horse thankfully!), and a small hard feed. At the moment all of mine get a handful of chaff and rice bran pellets. Rice bran is basically fat and oil. Not healthy in overly large doses but great for maintaining weight for growing horses, old horses, hard keepers, and horses over winter. All of my horses fit at least one of those categories. My OTTB who is nearly 15 gets the most rice bran, followed closely by my 24yo stock horse. My mare gets a bit more when she is having a growth spurt, but a small amount most of the time. I mix a powdered vitamin and mineral supplement through. It’s so much cheaper this way.
But realistically with horses, a lot of their behaviour comes down to their rider/trainer as much as their feed.
I would assume it would be similar with rabbits – Greebo certainly eats as much if not more than the bigger guys, but he is way, WAY more active. I feed Jack Rabbit as a bit of a fatty treat, but other than that they get lots of veg and 24/7 hay – the same hay I feed my horses and it is really good quality. The store bought hay I once had to buy when I ran out at night made Terry sneeze like crazy and none of them really liked it anyway, despite being “grown for rabbits.” I haven’t noticed any behaviour changes from the Jack Rabbit feed apart from less interest in veggies – so I never feed it until they have eaten their veg for the day. But then again, I don’t really feed that much of it because like I said, it’s more of a treat. They do really love it!
I’ll load some horse photos up for you in a sec.