Myconids are mutated fungi that spread noxious spores or charge at you. They come in several sizes. The larger ones may break apart into two smaller ones when killed. The smaller ones can grow into larger ones if left alone for too long. Myconid's that where broken off of a large Myconid will not spawn more Myconids even if they are allowed to grow into their larger forms.
Small Myconids can be easily killed by a slide or drop attack.