Most of the commentators understand ערוב as being derived from the word for "mixture", the animals being a "mixture" of a certain type. What type is subject to speculation. Shemos Rabbah (11:3) brings a difference of opinion between R' Nechemia and R' Yehuda as to what type of animals were involved: either insects or what we would think of as wild animals (like lions, leopards, bears etc.) respectively. [Incidentally, the midrash above (11:2) seems to say that birds were involved as well.]
What necessitates that the mixture be of animals specifically? The hint is Tehillim 78:45:
יְשַׁלַּח בָּהֶם עָרֹב, וַיֹּאכְלֵם; וּצְפַרְדֵּעַ, וַתַּשְׁחִיתֵם.
It has to be something that can "devour them". Wild animals make sense. Other commentators prefer more reptilian or rodent-like animals or other venomous creatures. (See Rashi and Abarbanel.) Insects that bite can also be considered to "devour" their victims, if there's enough of them. (See Shadal.)
[Also, see Bechor Shor, who compares "הִנְנִי מַשְׁלִיחַ בְּךָ וּבַעֲבָדֶיךָ וּבְעַמְּךָ וּבְבָתֶּיךָ אֶת הֶעָרֹב" (Shemos 8:17) with "וְשֶׁן בְּהֵמוֹת אֲשַׁלַּח בָּם" (Devarim 32:24).]
The only interpretation I know of that does not derive ערוב from "עירוב - mixture" is that of Rashbam, who derives it instead from "ערב - evening". He explains that the plague entailed a pack of wolves descending upon Egypt. (Maybe they were werewolves. :)) These wolves are called ערוב because they attack primarily at night.