I had 2 yellow h.reidi ponies,you have to make sure you get tank bred or there's very little chance that they will eat for you,the tank bred aren't extremely difficult but they need fed almost continuously because they have very small stomachs....I couldn't provide mine with a continuous supply of food without being home so I gave mine to a guy that did have time to devote to their health....once I retire from work,I will have horses again
they need a slow gentle water flow and cooler water than most reef tanks,they also do and look better in a tall tank rather than short tank and need plenty of hitching spots,sea fans and gorgonea are a welcome addition to their tank,they do well with mandarins too but fast aggressive eating fish are not good tank mates....
you can keep some coral with 'em too,no stinging coral,zoas do good in about any tank,macro algae is also another good option with horses,like in a display refugeum,they are a bit more difficult but if you have the time to devote to 'em,they are amazing creatures to watch and make a beautiful display,if you decide to take on that challenge, good luck to you
edit: I just noticed you said 5 gallon tank,I wouldn't use less than a 30g for a pair,5g is a bit small