arbeo : tree
The Illunse word for tree is arbeo. Arbeo is a last name. Arbeo of Freising (died 784) was a German Bishop.
This word is a mixture of the Latin word for tree which is arbor, and the Old English word for tree which is tréow.
arbeon : trees
The Illunse word for trees is arbeon.
Arbeon is a nominative case plural. The ending for this plural doesn't follow given that trees in Latin is arboris and trees in Old English is tréowu. It makes some sense if you change the grammatical gender of the noun tree, but never mind that. The -n ending is used for other Old English plurals.
I've changed these words. My Illunse word for tree is now tebor, trees is teboru.