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The term protogalaxy indicates something that is becoming a galaxy. The term is often used for a gas cloud showing signs of development in a galaxy, such as including some actual star formation. Such protogalaxies are expected to be frequent over a certain range of redshifts, given the number of galaxies that exist more recently. Their expected spectrum would be similar to a starburst galaxy, with hydrogen, helium, insignificant metallicity, and with electromagnetic radiation from the energy release of gravitational binding energy. Candidates are sometimes announced, but discussion generally remains regarding whether the cloud is indeed becoming a galaxy.
I believe the term protogalaxy may also be used to mean first galaxies, very-high-redshift fully-developed galaxies that are presumed to be among the very first that were formed, particularly if they show characteristics that distinguish them from more recent galaxies.