I think "transphobia" is like "porn"-hard to define, but you know it when you see it.
Example #1-way before I came out, one of my professors in university transitioned MtF. Some turned her into a butt of jokes, and one of her best friends ended the friendship

. I think about her a lot, ever since I came out years later.
Example #2-when I worked at the grocery, one of my regular customers was MtF. I knew immediately. While on break, some of my coworkers asked about her, if I knew she was "tranny", and did this make me uncomfortable? I replied that she had great ideas for preparing vegetables and her money was as green as mine

.
I've wondered about movies such as
Some Like It Hot, in which Jack Lemmon and Tony Curtis disguise themselves as women to avoid a mob hit. But to me, that's not about transsexuality; that's cross-dressing, a different animal altogether. Both of them play otherwise straight men forced into a ridiculous situation for a comedy.
We fear what we don't understand, so transphobia to me is fear of someone "different", and the ways that fear is expressed, through jokes, shunning, or gossip.