From what I can tell, whenever the word "Amon" - "עמון" appears in Tanach (not עמוני/ת, etc), it's always preceded by the word "bnei" - "בני", except for two instances1:
One in Shmuel 1:11:11 where it says:
"וַיְהִי מִמָּחֳרָת וַיָּשֶׂם שָׁאוּל אֶת הָעָם שְׁלֹשָׁה רָאשִׁים וַיָּבֹאוּ בְתוֹךְ הַמַּחֲנֶה בְּאַשְׁמֹרֶת הַבֹּקֶר וַיַּכּוּ אֶת עַמּוֹן עַד חֹם הַיּוֹם וַיְהִי הַנִּשְׁאָרִים וַיָּפֻצוּ וְלֹא נִשְׁאֲרוּ בָם שְׁנַיִם יָחַד."
and another in Tehillim 83:8:
"גְּבָל וְעַמּוֹן וַעֲמָלֵק פְּלֶשֶׁת עִם יֹשְׁבֵי צוֹר."
Why is this so? Why the difference from other verses with the word Amon?2
1 Thank you @ Kazibácsi for pointing out the second instance.
2 This is different from the question "Why “בְּנֵי עַמּוֹן”?" which asks why are they even called "Bnei Amon" in the first place. This question is about the reasoning for the two times in which they are not referred to as Bnei Amon but rather as "Amon".