Pesachim 90B

ואין שוחטין וזורקין על טמא שרץ. ועולא אמר: אף שוחטין וזורקין על טמא שרץ.

And one does not slaughter the Paschal lamb and sprinkle its blood on behalf of one who is ritually impure due to contact with a creeping animal, although he could immerse that day and thereby ensure he will be ritually pure for the evening. And Ulla said: One slaughters the Paschal lamb and sprinkles its blood even for one who is ritually impure due to contact with a creeping animal.

לרב, מאי שנא טבול יום ⁠— דחזי לאורתא? טמא שרץ נמי חזי לאורתא! מחוסר טבילה.

The Talmud discusses the rationale behind Rav’s ruling: According to Rav, what is different about one who is ritually impure with a type of ritual impurity such that once he has immersed himself during the day he will become fully ritually pure upon nightfall that, despite presently being impure, one may offer the Paschal lamb on his behalf? It is due to the fact that he is fit to eat it by night. But why, then, is the Paschal lamb not offered on behalf of one who is ritually impure due to contact with a creeping animal? He can also be fit to eat it by night by immersing in a mikveh. The Talmud answers: He lacks immersion in a mikveh.

טבול יום נמי ⁠— מחוסר הערב שמש! שמשא ממילא ערבא.

But one who immersed himself during the day and will become ritually pure upon nightfall also lacks the setting of the sun, i.e., nightfall. The Talmud explains the distinction: The sun sets by itself, and no action on the part of the one who is ritually impure is required to complete the purification process. A Paschal lamb may therefore be brought on his behalf.