Saprolings are fungus and/or plant-based, green-aligned lifeforms. They are capable of reproducing by spontaneous generation, though thallids and some treefolk have the ability to create them as well. "Saproling" seems to be a generic term for small plant/fungus creatures rather than a specific species. As a creature type, saprolings are used for tokens only.  There are about 60 cards that produce saproling tokens.
The first known saprolings on Dominaria were native to Sarpadia. They shared a symbiotic relationship with the thallids and spread rapidly by feeding off of the dead resulting from the thallid rebellion against the elves.
The saprolings of Yavimaya shared ties with the treefolk of the forest. They proved remarkably effective against undead during the Phyrexian Invasion, as they would drop out of the trees and burst open, spreading rotting agents over the Phyrexian zombies that would cause them to rapidly decay. As Yavimaya had been heavily involved in preparation for the invasion, it may be that their saprolings were specifically bred for that purpose.
Dominarian Saprolings (Time Spiral)
After decades of Dominarian desolation, the saprolings currently living on the world are different than those previously described. The present-time Dominarian saprolings are creatures made of thick kudzu vines, moss, algae and fungi.
Saprolings exist in the urban environments of Ravnica as well. They are much more plant-like than the fungal Dominarian saprolings, resembling crystals enveloped in floral tendrils.   They are particulary common in the Selesnya Conclave, where they bond with the guild's root-kin elementals. The Golgari Guild's saprolings are made of the bond between plants and rotting flesh that that guild seems to exemplify. However, they also are featured to a very minor extent in the other two partly-green guilds.
Saprolings are also present on Jund, one of the shards of Alara. They are created of plant matter, usually fungal, and have a strong connection with rotting corpses. They have a much different appearence than the other planes' saprolings, almost appearing shelled.
- Magic Arcana. (August 17, 2005.) "Ninth Edition Saproling Token Art", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (September 18, 2002.) "Ravnica Style Guide: Saprolings", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (December 13, 2005.) "Doubling Viashino", Daily MTG, magicthegathering.com, Wizards of the Coast.