First, define your environment.
Ecosystem does not only include creatures, but also the inorganic environment. Will it be a desert? A volcanic terrain? Sea? A lake with a lot of mana scattered around?
Second, establish a food chain.
Decide who will be eating who. Focus on the uniqueness of each of your creature and the similarity with earth's creature, or not.
A logical step would be the bigger creature eat smaller creature, but even earth has nature freaks, so get creative. I'm imagining a plant that specialize on eating ethereal wisps. Faeries that prey on centaur and other sentient creature.
Because you've introduced magic, then let's try to make magic as the primary source of energy, instead of sun.
Get something that feeds from magic, or mana. This is like "plankton" in our ecosystem. You can choose to have plants, but I recommend "spirit". They grow by absorbing mana from their surrounding. Other creatures cannot do that.
Get your first level consumer. Something that feeds on spirit. For example, "fairy". Imagine the spirit like a tiny floating fluffy ball and the fairies flying around consuming their energy. They do not "eat" them, but rather only takes the mana, so the spirit can still absorb mana again.
Get your second level consumer. I'll give you two examples: centaurs, and for creativity: Venus fairy-eater.
Proceed adding more creatures, define their relationship with each other.
Optional: for completeness sake, think about how to recycle energy in your world. Here, dead bodies are decomposed by detrivors, like bacteria, worms, cockroach, etc. How would that work in your world? Will they simply become ghost/spirit? Do they have physical body that can rot?
3It is really hard to answer your question without knowing anything about the magic of your world. Perhaps, you could give us more details about your world. – Olga – 2017-11-25T16:24:33.353
Sounds like Tha backstory for Pokémon – pojo-guy – 2017-11-25T16:56:44.117
So, will your ecosystem purely consists of mutated creatures and plants, or only some of them will occupy the new ecosystem? How different they will be from the creature they mutated from? – Vylix – 2017-11-25T18:21:45.293
2Building on @Olga's comment, we also need to know about your world before the impact. – JBH – 2017-11-25T21:35:24.983
1
Well, I expect magivore will be a niche.
– JDługosz – 2017-11-26T04:03:44.273