There are a number of wonderful zithers that could be confused with a guzheng. Below are examples, including some unknowns. If you recognize them or know of others we should add, please send in an email!
The guzheng in particular and past zhengs in general are often confused with three other instruments. They are the Japanese Koto, the Vietnamese Đàn tranh, and the Korean Gayageum. Below are wonderful examples of each.
This first image is a Japanese Koto from the 1800s (箏, こと, or alternatively 琴) photographed by the Museum of Fine Arts in Boston, Massachusetts. The instrument traces its lineage back to China's Tang Dynasty when 13-string zhengs were first brought to Japan. Their design has remained largely unchanged across time.
Big differences between the koto and a zheng are: Overall dimensions, string material and tying, and the tuning system. Kotos tend to be longer and thinner, though variants can certainly change this. Their strings used to be silk, with long loops of excess string left on the instrument to allow for retying when the strings break. Modern koto use a synthetic string made of either nylon or polyester; there is some confusion on its material composition in English sources. The modern tuning system is commonly held in the end cap like a zheng but is not enclosed in a box. See the second picture for an example.
Variants have been introduced with different string counts and lengths, such as the 17-string koto, but whereas in China where different instruments vied for popularity, in Japan they were accepted alongside the original 13-string instrument. The larger morphology remains steady - with the one exception of the 80-string variant made by Mitsuya Koto for Miyagi Michio, a renowned koto player:
Vietnamese Đàn tranh
Next of the three instruments most often confused with the zheng is the Đàn tranh. There is some debate as to whether its lineage is a branch of the Song Dynasty-inspired Chinese zhengs or if the Đàn tranh led to the Song Dynasty zhengs.
It's easiest to distinguish between modern long zithers, but harder to figure out the older ones. Modern đàn tranh have an opalescent styling, so if it's made in the last 100 years and features chromatic inlays and exposed soundposts, it is probably a đàn tranh. If it's older than 100 years or doesn't feature this styling, I can't help you.
Đàn tranh feature metal strings, bridges that have generally thinner legs, and I've seen a number with DRMSL labeling by the strings, representing the Do Re Mi So La notes of the pentatonic scale.
For a rare treat, check out this video by musicians Tri Nguyen and Qaïs Saadi which features plenty of footage and notes about this vietnamese zither.
Last of the three is the Korean Gayageum (가야금 or 伽倻琴) a traditionally 12 string zither from the peninsula. I know even less about this instrument and how its roots relate to the guzheng, but I can suggest that if a long zither is known to come from East Asia, has 12 strings, and features two straight, fixed bridges, the gayageum is a likely candidate.
But of course, using string counts to determine origin is far from guaranteed. #4 in this picture is itself a 25-string gayageum, while #1 and #3 have 12 strings. Bridges do tend to be more of a T-shape than a triangular style, but that isn't a guarantee either.
The site this picture comes from, the Korean Cultural Heritage Foundation, and a few other sources suggest the gaygeum traces its origins to the 6th century CE, but beyond that I can't say much. If you know more, send me an email!
Here's a treat. I don't know what this is. It comes to us from a MIMO entry from the Musée Du Palais Lascaris.
My French is terrible, but I discern it mentions 26 strings wound around 13 posts, possibly doubling back on themselves and comes from 1800s China.
Tapping into my knowledge of physics I'm guessing the posts by the fixed bridge are an anchor, the diagonal line of posts keep the string off the soundboard, and the line of posts at the top are the tuning pegs, possibly hand-wound based on the spacing and how they seem to taper flat, like the pin second from the right. But that's just a theory! It's 93cm/ ~36 inches long so a great deal shorter than confirmed zheng predecessors.
As for the doubling back, perhaps the were looking for an effect similar to today's 12-string guitars? Either that or it's supposed to be struck like a hammer dulcimer.
From the University of Leipzig we have, translated and paraphrased from the German record on MIMO:
A 12-string zither made in Korea between 1900 and 1950, this wooden body consists of a resonating soundboard, floor and frame. The soundboard is evenly arched. The 12 movable bridges are 6 of brighter and 6 of darker wood. The strings are wound copper and steel. There is a cut-out rectangular sound hole in the middle. Tuning pegs are hidden under one of the sides.
It's 83.2cm or ~32 inches long, so far shorter than most other long zithers from east Asia, and that, plus it being made in Korea, puts it in the unknown category. It may be a version of the Korean Gaygeum.
I could be wrong! A typical Koto, the Japanese long zither, is around 190cm, like this one at a US Museum. But that same museum has a miniaturized version that is just as old and comes in at 53.8cm. Perhaps this is a miniaturized Gaygeum?
Here's a strange one. This was taken at the Museum für Musikinstrumente der Universität Leipzig, in Leipzig, Germany. It appears to be 9 strings, with strings tied in the fashion of a koto. That is, the strings are left long and bundled on the tail, typically in loops. The tail is on the right side. The instrument is standing on its head, though I've rotated it here. The extra length of string is now hanging down the instrument.
Koto strings were left long to deal with silk strings breaking frequently. Since they'd usually break where they were played, by the head, you could thread the string towards the head until the break was even with your tying point, and tie it off again. Very little waste!
Because of that extra length and tie patter I'll suggest the strings are silk. I don't know why else they would have so much extra. As to date made or country of origin, I haven't the slightest idea.
Other Chinese Zithers:
This is a Se, not a Guzheng. It comes to us from the collections of the Museum of Fine Arts in Boston, MA, USA.
Believed to have been constructed in the 1800s it was purchased in 1916. I count 24 strings, which is one less than I expected, but there you have it. It comes in at a whopping 223.8cm or 88in long. It's far bigger than most other instruments on this page. Notice how it slants down at the tail. You'll see that in the Japanese Koto, suggesting this was inspired or connected to the designs of zithers in the Tang dynasty.
And we have another take on the Se, this time from The Met in New York, NY USA.
It's a bit shorter than the MFA Se, coming in at ~183cm or 72 inches, also dated to the 1800s, and also seems to have 24 strings, or 24 bridges, at least. This particular one was taken from a Taoist (Daoist) temple and is inscribed as being made by one Fanfu Lou and owned by the monk Xinzhu. Cheng 1991 suggests this was made in the Qing style of instrument, notable for its significant downward angle at one end.
More Se! This one has 25 strings and was on display at the Musical Instruments Museum in Brussels, Belgium.
The level of ornamentation on this is really fun. You have to realize, though, that weighty decorations can reduce the quality of the sound of the instrument, especially if they are on the soundboard. You need that wood to really vibrate, and any paint, inlays, or carvings will dampen or distort the sound. I'd guess this was constructed more as a work of art and less for its sound, but, I could be wrong!
A modern take on an older style of Se, as indicated by the knobs on the left of the instrument. This one was photographed in 2012 at the China Music trade show / expo in Shanghai. The text on the card translates to "18-string ancient se".
An this is supposedly a picture of the 50-string sè created for the Datong musical group in 1930's Shanghai, though I have yet to find singificant information on it.