Malbim Ayelet HaShachar Klal 277
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וכן השמות "עגל – פר" נקראו על שם גדולם; תחילה הוא עגל ואחר כך נכנס לכלל פר. והוא בשנה שניה לר' אליעזר ובשנה ג' לחכמים. (ויקרא ס' רח)
Ayelet Hashachar, translated by Betzalel Avraham Feinstein, 2021
And moreover, the nouns עגל - פר (male calf - bull) are used according to whether he is an adult; in the beginning, he is an עגל and afterwards, he becomes a פר. And it is in the second year according to Rabbi Eliezer and in the third year according to the sages.