With default configuration AltGr-e (the right Alt for those who have no AltGr) produces the generic currency symbol which looks like a four legged spider. When the font of the program is changed to a ISO-8859-15 font the currency symbol is replaced by the Euro symbol. In Debian this can be achieved by adding line
.XTerm.VT100.font: -jmk-neep alt-medium-r-*-*-*-120-*-*-*-*-iso8859-15
to the file /etc/X11/app-defaults/XTerm. The fonts
available in distributions and installations vary.
If AltGr-e does not work add line
keycode 26 = e E EuroSign
to the file /etc/X11/Xmodmap