"an animal of the class Mammalia; an animal that suckles its young," 1826, Englished form of Modern Latin Mammalia (1773), coined 1758 by Linnaeus for that class of animals from neuter plural of Late Latin mammalis "of the breast," from Latin mamma "breast," which is cognate with mamma. With the exception of a few egg-laying species, all bear live young and have the mammary gland for the young to suck. All also are warm-blooded and breathe air. In Middle English, mammille was "a woman's breast" (early 15c.).
variant of suffix -an (q.v.), with connective -i-. From Latin -ianus, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective. In Middle English frequently it was -ien, via French.
<a href="https://www.etymonline.com/word/mammalian">Etymology of mammalian by etymonline</a>
Harper, D. (n.d.). Etymology of mammalian. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mammalian