I am currently studying the declensions for nouns (currently on the 2nd one) and saw this difference.
amīcōs, fīliōs, agrōs
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Dōnum is neuter; amīcus, fīlius, and ager are masculine. Neuter nouns are always the same in both the nominative and accusative case, in both singular and plural. See this question for more about how universal this is.
Here are some more neuter nouns that you may be familiar with (from early chapters in Lingua Latina Per Se Illustrata):
Baculum dominī in mēnsā est. (Nominative)
Iūlius baculum suum sūmit et discēdit. (Accusative)
In vīllā sunt duo ōstia: ōstium magnum et ōstium parvum. (Nominative)
Vīlla duo ōstia et multās fenestrās habet. (Accusative)
Neuter pronouns are also the same in nominative and accusative:
Illud ōrnāmentum nōn tam pulchrum est quam hoc. (Nominative)
Dā mihi illud mālum! (Accusative)
Iūlius nihil ad hoc respondet. (Accusative)
Haec verba ā Lȳdiā nōn audiuntur. (Nominative)
Lȳdia haec verba nōn audit. (Accusative)
Quod ōrnāmentum est ānulus? (Nominative)
Ānulus est ōrnāmentum quod digitum ōrnat. (Nominative)
Ōrnamentum quod Lȳdia in collō habet est līnea margaritārum. (Accusative)
Quae ōrnāmenta ab Albīnō venduntur? (Nominative)
Ōrnamenta quae Albīnus vendit sunt ānulī. (Accusative)
This is because of the gender of the noun. Donum is neuter, wheras filius, amicus, etc, are masculine. The plural accusative for 2nd declension neuter nouns is -a.