What's up with animal names ending in る?

Question

猿・ツル・ホタル・カエル

る is a rather uncommon mora in nouns, especially at the end. But there seem to be an unproportional amount of animal names that end in る.

Is that just a coincidence?

For 猿 at least, there seems to be a theory that it's name is derived from a verb:

サルの語源には非常に多くの説があり未詳だが、中でも有力と考えられているのは、獣の中では知恵が勝っていることから「マサル（勝る）」の意味とする説。

-- 語源由来辞典

Answer 1

Here are a few etymology theories I've bumped into.

• サル -- Attested in Old Japanese. Not so likely to be from 勝【まさ】る. Might instead be from Ainu sarush or saro. Sar or sara means "tail", and -ush is a suffix that means "having". See also the relevant entries in Batchelor's Ainu dictionary. Both Gogen-Allguide and Nihon Jiten suggest a derivation from Chinese 猻 ("monkey"), but they each list the reading as sar, while the Wiktionary entry shows Old Chinese //*suːn//, Middle Chinese //suən//, modern Mandarin sūn and Cantonese syun¹ -- no sar.
• ツル -- Attested in Old Japanese. Appears to be a regional word. Compare also Korean 두루미 (durumi), Proto-Turkic *durunja (“crane”) (whence Turkish turna), Mongolian тогоруу (togoruu, “crane”), Hungarian daru (“crane”). See also the entry at Wiktionary.
• カエル -- Attested in Old Japanese. From Old Japanese form kaperu. As Naruto notes, possibly cognate with verbs kaeru ("to return") or kaeru ("to spawn"), both also from older form kaperu. See also the Wiktionary entries at 蛙, 帰る, and 孵る.
• ホタル -- Attested in Old Japanese. Also as Naruto notes, most likely 火【ほ】 (ancient reading only found in compounds) + 垂【た】る ("hanging", the way the bug's sparkle-butt hangs down). Some sources suggest that -taru here might be from 照【て】る, but the vowel shift makes this unlikely. See also the entries at Nihon Jiten and Gogen-Allguide (both in Japanese).

So only two of these are probably from verb roots.

Answer 2

Many theories indeed associate the names with various verbs, which can explain why they often end with る:

• サル ← 勝る
• カエル ← 帰る (to return)^[1]; 孵る (to spawn)^[2]
• ツル ← 連む (to flock) ^[3]
• ホタル ← 火垂る (for fire to drop) or 火照る (for fire to shine) ^[4]

But each names has several other theories unrelated to verbs. It is impossible to prove the exact etymology of such old Japanese wago, so no one may be able to answer this.