Twitter is not a neologism, but a word in the english dictionary: https://www.merriam-webster.com/dictionary/twitter ("to utter successive chirping noises"). As is tweet: https://www.merriam-webster.com/dictionary/tweet ("a chirping note"). My knowledge of trademark law is limited, but I guess if someone would like to sell "Twitter"-sausages, they would have to apply for that trademark in the respective category and it could be anyone, as the-company-formerly-known-as-twitter propably doesn't have the trademark for food?