Is there a way to implement a horizontal flatlist inside a vertical one after some data intervals?

It is possible to do this. It is about handling it in the outer FlatList's renderItem function.

A FlatList takes two functions, a renderItem and a keyExtractor. The keyExtractor is only necessary if your items don't have a key, and it is completely re-useable. So you will need a renderItem for every different FlatList that you plan on rendering.

So the outer FlatList uses the renderMainItem function, the inner FlatList uses the renderHorizontalItem

Notice how I have set up my data. Each object has a type that allows me to switch between what should be rendered in the renderMainItem function either returning a row or returning another FlatList.

Here is a snack showing it working https://snack.expo.io/S1sPDPAM4

import React from 'react';
import { Text, View, StyleSheet, FlatList, SafeAreaView } from 'react-native';

export default class App extends React.Component {

  constructor(props) {
    super(props);

    this.state = {
      data: [
        { type: 'row', text: 'row 1'},
        { type: 'row', text: 'row 2'},
        { type: 'list', data: ['Apple', 'Banna', 'Pear', 'Orange', 'Grape', 'Pineapple']},
        { type: 'row', text: 'row 3'},
        { type: 'row', text: 'row 4'},
        { type: 'row', text: 'row 5'},
        { type: 'list', data: ['Bike', 'Car', 'Train', 'Plane', 'Boat', 'Rocket']},
        { type: 'row', text: 'row 6'},
        { type: 'row', text: 'row 7'},
        { type: 'row', text: 'row 8'},
      ]
    }
  }

  renderMainItem = ({item}) => {
    if (item.type === 'row') {
      return (
      <View style={styles.mainItem}>
        <Text>{item.text}</Text>
      </View>
      );
    } 

    if (item.type === 'list') {
      return (
        <View style={styles.list}>
        <FlatList
          extraData={this.state}
          data={item.data}
          keyExtractor={this.keyExtractor}
          renderItem={this.renderHorizontalItem}
          horizontal={true}
        />
        </View>
      );
    }
  }

  keyExtractor = (item, index) => {
    return index.toString();
  }

  renderHorizontalItem = ({item}) => {
    return (
      <View style={styles.horizontalItem}>
        <Text>{item}</Text>
      </View>
    );
  }

  render() {
    return (
      <SafeAreaView style={styles.container}>
      <FlatList
        extraData={this.state}
        data={this.state.data}
        keyExtractor={this.keyExtractor}
        renderItem={this.renderMainItem}
      />
      </SafeAreaView>
    )
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: 'white'
  },
  mainItem: {
    height: 80, 
    justifyContent: 'center', 
    alignItems: 'center', 
    margin: 10, 
    backgroundColor: 'yellow'
  },
  horizontalItem: {
    width: 80, 
    justifyContent: 'center', 
    alignItems: 'center', 
    marginHorizontal:5, 
    backgroundColor: 'blue'
  },
  list: {
    height: 80, 
    marginHorizontal: 5
  }
});

This is just a simple example that shows one of many ways of creating a FlatList with a horizontal FlatList inside it.