AWK! Of course the ogre in Puss 'n Boots is not a troll! How could I have forgotten that the French ogre first appeared in that tale?!?
OK, now I don't know when exactly ogres in English became Trolls.
Now as regards Jenny Greenteeth, as far as I know (and I'm only an amateur so I could certainly be wrong), it's not that she shares an etymology with Grendel per se but that she is part of a tradition of freshwater amphibious humanoids who drag people underwater for food and fun.
I've done some more research since last post, since this was an interesting subject, and I saw that there was an earlier French mention of ogres than Perrault's: in Chrétien de Troyes' Perceval, le Conte du Graal (written between 1135 and 1190). However, these ogres are never described in any way aside from there being a land where they supposedly lived. From that I could infer either that ogres were monsters that were well known enough at that time and place to have needed no explanation, or else it was simply the name of a particular group of people, with no relation to the monsters.
However, in the book Romania by Paul Meyer et al., pp. 301-305, the author examines this passage from Perceval in multiple translations and manuscript variants, with an eye not only for determining its meaning, but also its authenticity. Meyer rebuts Heinzel's claim (in Ueber die franzosischen Gralromane) that the lines mentioning ogres are an interpolation, due to existing in multiple other manuscripts, and for other reasons. There is some possibility that the word simply meant "pagan", as it did in Middle Ages Dutch, but that doesn't necessarily mean that they weren't giving an already-existing monstrous appellation to a hated people.
The book also mentions that le Dictionnaire Darmesteter-Hatzfeld-Thomas claims that the earliest use of the word "ogre" is from 1527, but that the use in Perceval would predate that. Unfortunately, it's just not clear what the word actually meant at the time, but by Perrault's time, it had definitely acquired the meaning of a monstrous humanoid that could nevertheless pass for a human, which could hold positions of power, and had a taste for human flesh.
As for Jenny Greenteeth, I only noticed the similarity of the structure of the names: Green = Gren, teeth = del. The latter part is a bit of a stretch, but not too much with corruption and dialect variants, especially with the consonant d/t. Of course, there are many other folkloric creatures that match her description which have completely different names, like nyk / nökken / nykkjen, and all of the shapeshifting water-horse-men like the kelpie or each-uisge. Fascinating subject.
