Here is an attempted list:

1) Onen (Yevamos 70b compares Arel and Onen).

2) Tamei (see Rashi, Yevamos 70a, DH Ish, that an Arel is like a Tamei for eating Terumah).

3) Metzora (this is hinted at in Yevamos 70a where the Gemara derives "'Ish Ish' (Vayikra 23:4) to include the Arel." Since the verse itself mentions Metzora and does not mention Arel, we see that Arel is being compared to Metzora which is stated explicitly. See also the Tosfos ha'Rosh to Yevamos 71b, DH Arel. The Gemara on 71b states that an Arel can receive sprinkling. Tosfos (DH Arel) asks, why does the Gemara have to say this -- it is obvious! The Tosfos ha'Rosh answers that since "Ish Ish" compares Metzora to Arel, we might have thought that since Metzora does not receive sprinkling, so, too, Arel also cannot receive it. Therefore, it is necessary for the Gemara to state explicitly that an Arel can receive it. At any rate, we see that Arel is compared to Metzora).

4) Arel and Ben Nechar are compared (therefore, Yevamos 71a explains why the Torah has to write both of them -- because otherwise we would have thought that they are equivalent. See Zevachim 22b, which cites Yechezkel 44:9 which compares Arel and Ben Nechar).

5) An Arel who performs Avodah in the Beis ha'Mikdash is equivalent to a Kohen who sits down and performs the Avodah. (We saw above, in item 1, that an Arel is compared to an Onen. There is a further source for this in Sanhedrin 83a which states that if an Arel, an Onen, or somebody sitting down performed the Avodah in the Beis ha'Mikdash, he is not liable for capital punishment, but this is a Torah prohibition. We learn that an Arel is compared to a Yoshev.)

6) An Arel who serves in the Beis ha'Mikdash is compared to a Zar (non-Kohen) who serves in the Beis ha'Mikdash (see Rambam, Hilchos Bi'as ha'Mikdash 6:8, that an Arel who performs Avodah in the Beis ha'Mikdash receives Malkus in the same way that a Zar who performs Avodah receives Malkus).

7) Arel is also compared to Zav (see Tosfos to Zevachim 15b, DH Tevul, that Rebbi Akiva includes that an Arel is Tamei like a Zav and a Metzora).

8) Arel is like Mechusar Kipurim, according to one text (in Tosfos to Yevamos 74a, DH Hachi Garis, the Girsa is, "If an Arel or Mechusar Kipurim burned it, it is Pasul").

9) Arel is like a blemish. (See Rashi to Bereshis 17:1, that Hash-m said to Avram before he did Milah, "Walk in front of me and be perfect." Rashi explains that this means "as long as you have an Orlah you are a Ba'al Mum." See Or Zaru'a, Hilchos Milah #99, that an Arel is like a Ba'al Mum, and a Ba'al Mum is not allowed to serve for the Tzibur; see Menachos 9b and Rashi there, DH Zos Omeres.)

10) Arel is like a grave. (In the Mishnah in Pesachim 92a, Beis Hillel says that separating oneself from Orlah is like separating oneself from a grave. Therefore, if a Ger converted on Erev Pesach one cannot slaughter the Korban Pesach for him, just as one cannot slaughter the Korban Pesach for somebody who just touched a corpse. Such a person must wait seven days before he becomes Tahor.)

11) Arel may sometimes be like a rebel (see Chulin 4b, where a Beraisa says that everyone may slaughter, even a Kuti, even an Arel. The Gemara explains that the Arel referred to is a Mumar for Aralos. Rashi (DH Mumar l'Aralos) writes that this means he rebels against the Mitzvah of Milah, but this Beraisa maintains (at this stage of the Gemara) that a Mumar for one Mitzvah is not a Mumar for the entire Torah, only for that Mitzvah).

12) Arel is like one whose foreskin has been pulled back (see Yevamos 72a, that mid'Rabanan a Mashuch may not eat Terumah because he looks like an Arel).

13) In Yevamos 71a, there are three examples of circumcised people who are Halachically equivalent to an Arel. These are the Aravi Mahul, the Giv'oni Mahul, and the baby who was born naturally circumcised.

14) According to Tosfos in Yevamos 70a (DH ha'Arel), an Arel who cannot circumcise himself because his brothers died as a result of Milah, is compared to a Tumtum. The Gemara (72a) states that a Tumtum may not eat Terumah. His organs are covered up and he is not obligated to tear them open and do Milah. Tosfos (Pesachim 28b, DH Arel) writes that it is logical that he is not obligated to tear them open. The Har Tzvi (on the Chumash, end of Parshas Lech Lecha) writes that this is because a person is not allowed to injure himself. At any rate, an Arel is compared to a Tumtum. Both are not obligated, due to health reasons, to do Milah, but both may not eat Terumah.

15) The reason why the Arel is compared to all these unfavorable people that we have seen is that Rebbi Elazar ben Azaryah said in the Gemara (Nedarim 30b), "The foreskin is repulsive."

Kol Tuv,

Dovid Bloom