Help me with strange sentence.

It's not a full sentence, but like a caption. It's not anymore unusual than captioning a picture or video you post with "my dog eating a meatball". The only difference is this is a meme or joke being made, and it's just a representation of his first follower thinking he forgot them. Rather than a literal video of the follower. And I take it there is also something to show that the man has not forgotten.

This isn't really a proper sentence; it's describing a situation, like you might see in the caption for an image. \[implied: This is a picture / video / face made by...\] "my first follower thinking I forgot him"

Just to add to the correct posts that identify this as a caption: This was a thing long before the internet. For example, in an encyclopedia, you could have an article about carnivores. To illustrate the concept, there could be a photograph of lions eating a wildebeast. The caption to this image would likely read something like "Lions eating a wildebeast". The rest of the sentence is omitted for economical reasons and simply left implied.

This is a reduced relative clause modifying the noun "follower". It's also just a sentence fragment, so the verb is missing. The complete formal sentence would be something like: This is my first follower who is thinking that I forgot him. In subject relative clauses that use "be" and a participle, we often drop the relative pronoun and the verb. The meme also drops the subject and verb, leaving only the noun and clause left, because its the internet and people drop whatever they can to be concise. I have a dog. He is sleeping on the floor. I have a dog that is sleeping on the floor. I have a dog sleeping on the floor. A dog sleeping on the floor < your meme's structure

I think he is saying that this is from the point of view of the posters first follower thinking that the poster has forgotten them. And the video would be the first followers actions because they think the poster has forgotten them. A better way to word it would be: “My first follower thinking I forgot him” —> “My first follower thinks that I have forgotten him” (implying that he has not forgotten his first follower, and will subsequently show that he has not forgotten them)

It’s a fragment. Imagine the whole statement being “[This is my impression of] my first follower thinking I forgot him”

"Thinking" here is a participle, in this case acting as an adverb. For example: I am looking at the sky. I thought about her, looking at the sky. In the second example, "looking" is describing what the subject is doing during the verb of the sentence. In the context of your post, there is no given verb, but the implication is that the verb is whatever the follower is being shown doing.

[this is] my first follower thinking I forgot him

It's missing a lot of context. Looks like he's an online influencer. So the full phrase is: "My first \[online\] follower \[is\] thinking \[that\] I forgot \[about\] him. He omitted these words to keep it short.

it's a caption structure (not a standard sentence structure) of A. [subject], B. [emotion/descriptive quality] and then C. [the reaction or result] in this example: - A: my first follower - B: thinking I forgot about him - C: whatever this guy is playing out in the video. he's essentially playing the role of "my first follower," and acting out the scenario. you also see this written like this: [imagine a post about bad cooking] "professional chefs (A) reading this post (B)" followed by a gif of someone being horrified (C). it's the same structure.

He’s referring to his followers online

It is right, or at least natural to read. My formal grammar sucks ass for a native speaker. My first follower thinking I forgot him. Subject: [My first follower] verb: [thinking] adverbial element: [I forgot him] it's the immitated face of a follower who believed they were forgotten.

I can't speak to grammatical structure necessarily but I think the people saying it's wholely wrong might be incorrect? Or at the very least for the wrong reason. I see this structure in older literature. This caption is just like part of the sentence is cut off. So here's my best examples. *My first follower, thinking I forgot about him, unfollowed me because he felt unappreciated.* *My first follower, thinking I forgot about him, messaged me to ask if I remembered him.* These sound right to me. It's a bit stuffy and old fashioned, but this structure is meant to be followed by an action of some kind. The implication in the video is that the follower is sad. There is a bittersweet emotion here. The follower is proud of/happy for the creature for growing, but feel left behind and unimportant. So it's like, "My first follower, thinking i forgot about him" followed by the person in the video doing an action, role playing as the follower. The action part of the sentence is cut off because the creator is *showing you* the action instead of writing it down. You see this all the time in memes and shorts on YouTube and TikTok. (Person/animal) (is feeling/thinking/doing): image or video of (person/animal) performing an action because of the (feeling/thought/action). Does that make *any* sense?

This is a complete sentence. There are many ways to structure this. Some of the others shown in the comments would be more formal or follow older grammar rules, but there is nothing wrong with the structure here either. The noun is a compound “My first follower” and the verb “thinking” is a present participle, indicating an ongoing action in this case. English has many ways to say things that all have nuance and regional/generational preferences.

Internet language,i guess (people don't bother using helping verbs nowadays).