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beathachanimalmasculine noun
the
am beathach
after a preposition
a' bheathach
of the
a' bheathaich
plural
beathaichean
the (plural)
na beathaichean

Every form but the headword, the genitive and the plural is worked out by the spelling rules in the grammar guide, so what you see here is what the guide teaches. Checked by a fluent speaker.