I did want to mention that technically, the female seahorse is the one giving birth, she's just laying her eggs into the pouch of the male. To my knowledge the male isn't secreting any food or nutrients to the eggs, theyre developing on the yolk provided by the female. They're just being sheltered by the male.
A more accurate simulation might be for the female to lay eggs into the male's inventory, where he will then carry them until they hatch. This could also be used to simulate parasites like wasps that lay their eggs on/in a particular host, but in the seahorses' case its the host's intention to carry the eggs, like with some sort of "WILL_SHELTER_EGGS" tag, while in parasitic species the victim can attempt to dislodge them/wash them off if the eggs are external and its not paralyzed, or a doctor could remove deadly chestbursters from unfortunate dwarves. So a setup where a female/CAN_BIRTH lays eggs, they get an additional [EGG_LAYER_TARGET_CREATURE:(creature_ID):(CASTE_ID)] or EGG_LAYER_TARGET_CLASS:(class)] tag, and a TARGETED_EGG_INTERNAL/EXTERNAL flag. Pregnant egg layers with these tags will then seek out a valid target and lay their eggs on/in them, or lay eggs on themselves like how wolf spiders carry their eggs on their backs. The eggs end up in the creature's inventory, and as long as they stay in its inventory they're considered to be incubated properly. If the creature doesn't have a [WILL_SHELTER_EGGS] flag then it will attempt to remove them/a doctor will remove them. If plants could be targeted, then giant wood boring insects or giant moths/butterflies could seek out a plant of an appropriate species to lay their eggs on. Could go even farther to say, give creatures like giant cave spiders the ability to make a cocoon out of their silk and lay their eggs in that, or birds/reptiles can make a nest out of branches/dirt, placing it in appropriate areas like on a target plant or tree, which counts as a natural nest box. Such cocoons/nests could be encountered in the wild/made by wildlife in-game, and then provide natural sources of eggs and tameable hatchlings if the player dares steal them.
But then, whether or not the hatchlings eat whatever their host is, can be simulated some other way. Maybe they attack the host? Destroy tree tiles? I dunno.