Melani(cat)- basically means black or dark... it fits perfectly.

Ethan(dog)-"solid, enduring" in Hebrew.

Otto(rat)- his was the name of kings of Germany, including Otto I, the founder of the Holy Roman Empire, who was known as Otto the Great. A king of Greece also bore this name. (thats all I could find)

Venny(rat)- Basically, its a shorter version of Vincent, and spelled differently, and Vincent means: From the Roman name Vincentius, which was from Latin vincere "to conquer".

Ophelia(rat)- Derived from Greek οφελος (ophelos) meaning "help".

Pocky(rat)- Its food.. basically a chocolate covered biscuit stick(comes it different flavors and such). XD

Aiko(rat)- Means "love child" in Japanese.

Marlee(mouse)- From a surname which was taken from a place name meaning "pleasant wood" in Old English. A famous bearer of the surname was the Jamaican musician Bob Marley(my hero^^).

Hinge(betta)- those bendy things that hold doors to the frame.

Clyde(betta)- From the name of the River Clyde in Scotland, which is of unknown origin.

Faye(betta)- Named after my mum's middle name, but other than that: Derived from Middle English faie meaning "fairy".

Parka(betta)- A coat or jacket with a hood and usually a warm lining for cold-weather wear.

Reich(goldfish)- The territory or government of a German state, as the Holy Roman Empire, or First Reich, from the ninth century to 1806; the German Empire, or Second Reich, from 1871 to 1919; the Weimar Republic, from 1919 to 1933; or the Third Reich, from 1933 to 1945.

Haiku(Ryukin)- A Japanese lyric verse form having three unrhymed lines of five, seven, and five syllables, traditionally invoking an aspect of nature or the seasons.

The others, I couldn't find the meaning, or they really just don't have a meaning.