English has many examples of portmanteau words (e.g. "motel" is a combination of "motor" and "hotel"). Does Latin have any such phenomena?
Here is one candidate for a Latin portmanteau: hodie.
The word means roughly the same as hoc die and is believed to come together from these parts somehow. We don't exactly know how; the exact origin is unclear. One way to interpret it is to see it as a portmanteau of hoc and die so that some of hoc is lost when smashed together with die. (The lost bit would the c and half the length of o.)
Whether this qualifies for an actual portmanteau is perhaps a matter of taste, but it is such a common word that comes so close that I wanted to bring it up.
Non in Latinitate Antiqua invenitur, sed quidam @Atticist Gratiactio verbum abhinc minutos proposuit:
This actually came up during an answer to a question I had asked, and the word from that answer was saxifragus, or "rock-breaking". I'm not sure how common it was practiced as compared to languages that do it more broadly (I'm thinking of English and German as examples), but it does seem to be attested to some extent.