They're like mutated versions which are optimised for certain tasks. A bit like how a bee colony automatically spawns different types of bees (queen, worker, drone) depending on the situation. Morph details are here.
You can obtain them by assigning 1 to 3 other throdamas to join a pregnant one, or just the baby (I can't remember if it's too late by then). They'll look after it (be busy the whole time) and use up their pheromones on it. The baby will grow into a certain morph as an adult.
Every worker (normal) throdama has 4 types of pheromones, listed on their pages. You have to unlock them by doing tasks with them. They have one use each, and to get better grades of morphs (more abilities) it can cost up to 3 others' pheromones. So they're quite rare and expensive.
I recommend getting at least one of each morph, because they all pop up in quests sometimes. And you might as well make them grade 3, if possible, because the extra abilities make them way more useful/strong.