Yeah, but that's the point. Armoured cav replaced horses. For horses to return the battlefield (as cavalry, rather than in a special forces role), they'd have to offer something material that tanks don't. And within the constraints of real life genetic engineering, they cannot.

I suppose strictly speaking, the question is "could they survive on a modern battlefield"... to which the answer is still no, because anti-cavalry weapons are currently designed to eliminate tanks.

Technically, Cataphracts are armored cavalry

What about other factors? Like how they don't need fuel. How their maintenance requires no highly skilled labor and machinery. Or how you could use them to enhance non-mechanized units rather than replace existing vehicle units? Even the Germans in WWII realized that soldiers on bicycles was superior as long as flat roads were available compared to having them walk.

Horses do need fuel. In a harder to transport form than gasoline too.

The delusion that horse maintenance requires “… no skilled labor or machinery…” is laughable. Horses require doctors for maintenance.

@DanielB they require doctors for repairs, not maintenance. Assuming ofcourse they are engineered with genetics that few viruses can make use off, which would be wise. Even then it would likely be easier, as you can engineer the horse to react to specific chemicals to make operations easier, if it even comes to an operation. "Horse 6 broke its leg". "Well just saw it off at the usual spot, nail one of the spare legs to it and let it rest for 2 months while we rotate it out".

@Demigan - This genetic engineering is limited by real world constraints. You're well into "magic genetics" at this point. And as any stable can tell you, you need doctors for maintenance.

Also worth noting, the OP specified "cavalry". Not a means of making infantry more mobile.

@jdunlop what magic genetics? I'm not adding superstrength, I'm using a very unique genetics structure to make it hard for virusses to invade, just like many pig diseases cant jump to humans because of the different genetics. For any virusses that can still infect them you can make a database of antivirusses from the horses own immune system and incorporate that into the horses so they are immune to any old strain, making it hard for any new strain to develop. The ability to place more sphincters at bloodvessles to close them off in case of wounds and amputations helps as well

@jdunlop with added methods to ensure a low rejection of new organs and modified regenerative abilities you can ensure a good healing process. Just like if you accidentally cut off your finger it can be reattached if you are quick. It will lose some functionality because our bodies arent made to deal with reattached limbs but these horses could be made to do that. Also the OP uses "cavalry" as "people on horseback". He specifically mentions "could there be a way to modify horses to make them useful again?". And yes, there are.

The "spitting acid" part would also make the horses violations of the Chemical Weapons Convention.

@Demigan: it might be worth asking about the plausibility of those things on the biology Stack Exchange.

@RobinSaunders disease protection should not require asking. We've been doing that for our crops for decades now, with the only big caveat that companies only build a few genetic strains because of economic reasons meaning if a disease adapts to one strain an entire crop can go to waste. The simple addition of hundreds to thousands of strains can reduce that. Some animals can also practice self-amputation, which would be a valuable trait for making amputation easier, and animals can also regenerate entire bodyparts, which could be adapted to "bridge the gap to this new bodypart".

If "disease protection" was an easy magic bullet type of thing, I think that the world would be very different right now.

There's a huge gap between "we can protect against disease" and "we can guarantee effective resistance to any pathogen strain we have knowledge of, even to the point where dangerous new strains arising is unlikely". (A population being immune to a strain doesn't mean it can't be carried or transmitted, or that it can't evolve.) There's also a huge gap between "some animals can do this" and "we'll be able to transfer this ability to any other animal of our choosing". Despite some elevator pitch accounts, genetic abilities in general are not simply mix-and-match.

For that last bit, I suggest Googling "genes are not blueprints".

Lots of the contradictions to this answer seem to be arguing that horses could assist infantry that don't have transport, and so it behooves me to add: No first world army, currently (let alone in gene engineering land) is fielding any unmechanized troops. From helicopter transport, down to trucks and at a squad level, jeeps; a vanishingly small number of people are going anywhere on foot. (And those that are would be compromised by the presence of a horse.)

@Demigan - RobinSaunders covered the bases for me in terms of magic genetics. Immunity against all/most pathogens? Regeneration? Extra valves in blood vessels? CRISPR/Cas9 can't do that.

For all the people who think its magic genetics. I said it would heavily reduce the diseases. Its simple math: if you have 600 diseases that can be transmitted through your population you have more members sick and more chances of more dangerous or dibilitating strains appearing than a genetically altered version that has about 30 diseases left that can make it sick. It also makes it easier to use quarantaining to reduce spread once a disease does pop up. The DNA does control indirectly the shape and structure of a body, so using existing traits should be possible.

To deny that you might as well claim that a change in DNA is meaningless and that mutations do not exist. Its not as if the placenta was a piece of virus DNA left in mankind that became active due to mutations and offered us a great way to feed and grow babies now eh? Nope that never happened! An entire bodyplan of nerves, bloodvessles and flesh was not activated due to a specific set of genes creating hormones and chemicals at specific intervals in the body! Nope! It must have been a magic dude in the sky, DNA does nothing! Yes this magic "I'm against it so it cant be true" thinking irritates

I don't think anyone is questioning that protection against more diseases leads to less sickness. We're questioning the plausibility of genetic engineering providing the level of protection you're describing.

I'm sorry for irritating you, but presenting a strawman of what we're saying isn't going to persuade anyone.