Though typically spelled differently in Japanese, Fafnir appears to be the intended name. Credit to Kahran042 for the suggestion.

Ropross is a large robotic pterodactyl from Mitsuteru Yokoyama's manga series Babel II.

Probably the toughest randomly encountered monster in the game, but it's also worth more experience points than anything else by a sizable margin, and is one of very few sources of the elusive and awesome Three Stars accessory, not to mention extra Ribbons.

The name is possibly a cross between brachiosaurus (a type of dinosaur, with its name derived from the Greek for "arm lizard") and raidus (supposedly Greek for "raid"). Yu-Gi-Oh! has a card with the same name, for what it's worth, though the English verson came out as "Bracchio-raidus" for whatever reason.

Thanks to Kahran042 for suggesting the extinct "elephant bird" for the name.

Some search results link バセジアータ with the spelling "vasegiatta" and the word "harpy", but I haven't been able to determine whether that connection is meaningful or only exists because this monster is called Harpy in the earlier English releases and Vasegiatta in the GBA version. Either way, though, it works as a decent rendition of the phonetics, whether it necessarily means anything or not.

Possibly named after an Italian municipality. There's also a Zoka Coffee that matches the pronunciation, but the game predates it by about two years.

Gau has this Rage already learned when he initially joins the party.

When fighting these, note that they're both unusually resistant to physical attacks and unusually vulnerable to magic.

Or something along those lines. The only part of the name I can really make sense of is the "meadow", and I can't even be sure whether that part was intentional. There may be a connection to the foul-smelling Rafflesia flower, though the phonetics don't quite match up, and I've also seen speculation on a Japanese site that it might be "low flesh" instead, not that that seems to make any sense.

Name transliteration based on プレ = pre- and ランタール = Lantaru, a sector of space in Star Trek.

Very high defenses, but very little health. Defense-ignoring attacks make short work of them, including healing spells since they're undead.

Gau has this Rage already learned when he initially joins the party.

Monster formations fought only in the coliseum don't appear on the Beast Plain, so Gau can never legitimately learn this Rage, though it's questionable whether Flare and innate 'protect' are enough to make up for being weak against every element, anyway. Might be good against pure fighters.

Be careful about fighting him. Coming from his respectable level and magic power, Hyperdrive in particular is brutal, inflicting around 7,200 damage on average.

Unlike the other monsters in the Lethe River, Civil Amor must always appear in at least one of the forced battles.

Hroþgar SCRB notes that this could also be the name Sybil Amor.

Gau has this Rage already learned when he initially joins the party.

As Hroþgar SCRB points out, ディ phonetically fits the Italian word "di", roughly equivalent to English "of".

Could also be "leech" or "leach". There's not much to go on, but they do like to Jump, so maybe it's for reaching high heights?

One of the odder names, but something interesting happens if you break it up as リト + ワール + ビッヘ. リト is トリ (= bird) backwards, ビッヘ is an exaggeration of ヘビ (= snake) backwards, and ワール is similarly an exaggeration of ワル, which means to split or divide. In other words, it's basically bird (backwards) ÷ snake (backwards). I've tried to do something similar in English. The ッヘ should have tipped me off that something odd was going on; I'm not sure I've ever seen that combination anywhere else.